This commit adds support to the makefiles, configuration script, and build
system to understand MUSL. This is broken up into a few parts:
* Any target of the form `*-musl` requires the `--musl-root` option to
`./configure` which will indicate the root of the MUSL installation. It is
also expected that there is a libunwind build inside of that installation
built against that MUSL.
* Objects from MUSL are copied into the build tree for Rust to be statically
linked into the appropriate Rust library.
* Objects for binary startup and shutdown are included in each Rust installation
by default for MUSL. This requires MUSL to only be installed on the machine
compiling rust. Only a linker will be necessary for compiling against MUSL on
a target machine.
Eventually a MUSL and/or libunwind build may be integrated by default into the
build but for now they are just always assumed to exist externally.
This pulls all of our long-form documentation into a single document,
nicknamed "the book" and formally titled "The Rust Programming
Language."
A few things motivated this change:
* People knew of The Guide, but not the individual Guides. This merges
them together, helping discoverability.
* You can get all of Rust's longform documentation in one place, which
is nice.
* We now have rustbook in-tree, which can generate this kind of
documentation. While its style is basic, the general idea is much
better: a table of contents on the left-hand side.
* Rather than a almost 10,000-line guide.md, there are now smaller files
per section.
In the past, windows was installed from stage3 to guarantee convergence between
the host and target artifacts, but syntax extensions on all platforms are
currently relying on convergence, so special casing this one platform has become
less relevant over time.
This will also have the added benefit of dealing with #13474 and #13491. These
issues will be closed after next next nightly is confirmed to fix them.
You rarely want to statically link against librustc and friends, so there's no
real reason to install the rlib version of these libraries, especially because
the rlibs are massive.
