Breaking posix_memalign precondition is not UB
The `size==0` test here might be overtesting, but I figured might as well test it and leave a comment saying it is fine to remove it if the implementation changes.
Fixes#2099
add special exception for std_miri_test crate to call std-only functions
These being the unit tests of std, they have their own copy of `std::sys` and `std::thread`, so the existing check says this is not std. The check is correct but we want to allow this so we just hard-code the crate name.
The point of this `frame_in_std` check is to prevent people from directly interacting with shims that aren't really properly implemented, but it doesn't need to be 100% airtight. If someone really wants to call their crate `std_miri_test` in order to access some broken shims... they can keep the pieces.
fix RUSTC_BACKTRACE always being set
I kept wondering why Miri programs, whenever isolation is disabled, behave as if RUSTC_BACKTRACE was set. Finally I realized it's because some early rustc setup code sets that env var, and that is then propagated to the interpreted program.
So fix that by taking a copy of the environment before any rustc setup, and use that copy as the basis for what is provided to the interpreted program.
implement some missing float functions
With this we support the entire float API surface of the standard library. :)
Also fixes https://github.com/rust-lang/miri/issues/2468 by using host floats to implement FMA.
Some crates are using nightly and failing when mapping these errors,
for example <https://miri.saethlin.dev/?crate=remove_dir_all&version=0.7.0>:
```
error: unsupported operation: io error NotADirectory cannot be translated into a raw os error
--> /root/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/sys/unix/fs.rs:1203:19
```
fix build.rs invoking RUSTC to do check builds
This makes the Miri driver, when invokved via the RUSTC env var from inside a build script, behave almost entirely like rustc. I had to redo how we propagate sysroot information for this (which is actually back to how we used to do sysroot propagation many years ago).
Fixes https://github.com/rust-lang/miri/issues/2431
this requires a change in sysroot handling: miri driver now requires
MIRI_SYSROOT to be set when it is in 'target' mode, rather than relying on
`--sysroot` always being present.
