Provide pre-build host compiler for aarch64-unknown-linux-musl

[alex]

aarch64-unknown-linux-musl is currently a tier 2 platform with pre-built libstd, but not pre-built host compiler (rustc, cargo, etc.) It would be useful to me if the host compiler was available. Specifically it's important to me because pyca/cryptography -- the most widely used Python crypto library -- builds and tests on Alpine Linux, which uses musl. We're currently looking at expanding aarch64 testing in CI, and not needing to special case Alpine aarch64 would be great.

I'm happy to try to work up a PR for this, but there's one thing I'm not positive about: can this be a cross-compilation, the way aarch64-unknown-linux-gnu is (I know there's ongoing work to have aarch64 be tier1 with tests run natively).

I've currently hacked up the dockerfiles to do this, but I'm only about 40% sure I did it correctly, so we'll see, assuming this idea is acceptable.

[alex]

Fixed in #76420