Copy source files from rust code

Add missing wildcard

Remove unused function

Remove use of tar --transform

This is to pull in changes to support ARM MUSL targets.

This change also commits a couple of other cargo-generated changes
to other dependencies in the various Cargo.toml files.

The compiler-rt build system has been a never ending cause of pain for Rust
unfortunately:

* The build system is very difficult to invoke and configure to only build
  compiler-rt, especially across platforms.
* The standard build system doesn't actually do what we want, not working for
  some of our platforms and requiring a significant number of patches on our end
  which are difficult to apply when updating compiler-rt.
* Compiling compiler-rt requires LLVM to be compiled, which... is a big
  dependency! This also means that over time compiler-rt is not guaranteed to
  build against older versions of LLVM (or newer versions), and we often want to
  work with multiple versions of LLVM simultaneously.

The makefiles and rustbuild already know how to compile C code, the code here is
far from the *only* C code we're compiling. This patch jettisons all logic to
work with compiler-rt's build system and just goes straight to the source. We
just list all files manually (copied from compiler-rt's
lib/builtins/CMakeLists.txt) and compile them into an archive.

It's likely that this means we'll fail to pick up new files when we upgrade
compiler-rt, but that seems like a much less significant cost to pay than what
we're currently paying.

cc #34400, first steps towards that

Bring the calculation logic in line with the makefiles and also set the
RUSTC_BOOTSTRAP_KEY environment variable to enable the bootstrap on the stable
compiler.

Not much new, just bringing everything along.

* Fixes a warning with libc
* Brings in some new flag updates for various platforms through gcc-rs
* Otherwise routine updates here/there

This commit is the start of a series of commits which start to replace the
makefiles with a Cargo-based build system. The aim is not to remove the
makefiles entirely just yet but rather just replace the portions that invoke the
compiler to do the bootstrap. This commit specifically adds enough support to
perform the bootstrap (and all the cross compilation within) along with
generating documentation.

More commits will follow up in this series to actually wire up the makefiles to
call this build system, so stay tuned!