Gather code review data to help guide the code review process. This is currently limited
to i686 and x86_64 Linux because these are the targets I could get working. More
investigation is needed to figure out how to collect coverage data for ARM targets.
Avoid requiring a sysroot for *-linux-musl targets when using Clang.
Add one AAarch64 and one 32-bit ARM MUSL target to GitHub Actions.
Use Rust 1.48's `-Clink-self-contained=yes` in CI for musl targets.
Support the non-default variants of the *-musl targets.
Android has its own `clang`, `llvm-ar`, etc., and if they are in `$PATH` then
they might accidentally get used when it isn't intended for them to be used.
As part of this, reduce the environment variables that are set to (almost) only
the ones needed for the given target. An exception is wasm32-unknown-unknown,
which has its variables set even regardless of whether "wasm32_c" is used.
