Musl host toolchain
Based on https://github.com/rust-lang/rust/pull/55163 and https://github.com/rust-lang/rust/pull/57359
Depends on https://github.com/rust-lang/rust/pull/55566
CC https://github.com/rust-lang/rust/issues/57439
### How it works
Tested compiler made by `dist` on glibc and musl based distributions and verified binaries it produces:
* Ubuntu (glibc) - installed it as a target for host toolchain and observed no regressions for static (default) linking, dynamic linking apparently requires musl build libgcc so I didn't test it.
* Alpine (musl) - installed as the host toolchain, by default it links statically (executables are portable and work on glibc distributions) but with `-C target-feature=-crt-static` Rust flag it links dynamically (executables require musl built libraries).
### What's debatable
It should be decided whether this toolchain should link dynamically or statically when using it on musl distribution. I believe the distributions would prefer dynamic linking but it'd be misleading because `$ARCH-unknown-linux-musl` target links statically on the other hosts.
Another problem is using `RUSTFLAGS='-C target-feature=-crt-static'` for dynamic builds which is really uncomfortable.
To address both issues I suggest leaving `$ARCH-unknown-linux-musl` static for both host and cross target and introducing "alias triple" `$ARCH-unknown-linux-dynmusl`. It'd be the same as `$ARCH-unknown-linux-musl` (and use the same libraries to avoid duplication) but it'd link dynamically.
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### Why it's still WIP (help wanted)
I'm having a hard time getting all tests to pass and I'd appreciate help.
Non-verbose error:
<details>
```
Testing proc_macro stage1 (x86_64-unknown-linux-musl -> x86_64-unknown-linux-musl)
Compiling proc_macro v0.0.0 (/checkout/src/libproc_macro)
error[E0463]: can't find crate for `std`
error[E0463]: can't find crate for `std`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0463`.
error: aborting due to previous error
For more information about this error, try `rustc --explain E0463`.
[RUSTC-TIMING] proc_macro test:true 0.529
[RUSTC-TIMING] proc_macro test:false 0.530
error: Could not compile `proc_macro`.
warning: build failed, waiting for other jobs to finish...
error: Could not compile `proc_macro`.
To learn more, run the command again with --verbose.
command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-musl" "-j" "16" "--release" "--locked" "--manifest-path" "/checkout/src/libtest/Cargo.toml" "-p" "proc_macro" "--"
expected success, got: exit code: 101
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test --host x86_64-unknown-linux-musl --target x86_64-unknown-linux-musl
```
</details>
Verbose error:
<details>
```
Testing proc_macro stage1 (x86_64-unknown-linux-musl -> x86_64-unknown-linux-musl)
Compiling proc_macro v0.0.0 (/checkout/src/libproc_macro)
Running `/checkout/obj/build/bootstrap/debug/rustc --edition=2018 --crate-name proc_macro src/libproc_macro/lib.rs --color never --crate-type lib --emit=dep-info,link -C opt-level=2 -C metadata=09ddd3ecc930ab63 -C extra-filename=-09ddd3ecc930ab63 --out-dir /checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps --target x86_64-unknown-linux-musl -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/release/deps -C target-feature=-crt-static`
Running `/checkout/obj/build/bootstrap/debug/rustc --edition=2018 --crate-name proc_macro src/libproc_macro/lib.rs --color never --emit=dep-info,link -C opt-level=2 --test -C metadata=a564d363930469c8 -C extra-filename=-a564d363930469c8 --out-dir /checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps --target x86_64-unknown-linux-musl -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/release/deps -C target-feature=-crt-static`
error[E0463]: can't find crate for `std`
error[E0463]: can't find crate for `std`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0463`.
error: aborting due to previous error
For more information about this error, try `rustc --explain E0463`.
[RUSTC-TIMING] proc_macro test:false 0.248
error: Could not compile `proc_macro`.
Caused by:
process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc --edition=2018 --crate-name proc_macro src/libproc_macro/lib.rs --color never --crate-type lib --emit=dep-info,link -C opt-level=2 -C metadata=09ddd3ecc930ab63 -C extra-filename=-09ddd3ecc930ab63 --out-dir /checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps --target x86_64-unknown-linux-musl -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/release/deps -C target-feature=-crt-static` (exit code: 1)
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] proc_macro test:true 0.248
error: Could not compile `proc_macro`.
Caused by:
process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc --edition=2018 --crate-name proc_macro src/libproc_macro/lib.rs --color never --emit=dep-info,link -C opt-level=2 --test -C metadata=a564d363930469c8 -C extra-filename=-a564d363930469c8 --out-dir /checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps --target x86_64-unknown-linux-musl -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-musl/stage1-test/release/deps -C target-feature=-crt-static` (exit code: 1)
command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-musl" "-j" "16" "--release" "--locked" "--manifest-path" "/checkout/src/libtest/Cargo.toml" "--verbose" "-p" "proc_macro" "--"
expected success, got: exit code: 101
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test --host x86_64-unknown-linux-musl --target x86_64-unknown-linux-musl
```
</details>
Whole tests non-verbose output: [rust-tests.log](https://github.com/rust-lang/rust/files/2879945/rust-tests.log)
I think the error is because build system (correctly?) tries to use `obj/build/x86_64-unknown-linux-musl/stage1-test/x86_64-unknown-linux-musl/release/deps` which is empty but `obj/build/x86_64-unknown-linux-gnu/stage1-test/x86_64-unknown-linux-musl/release/deps` contains required libs.
</del>
