RFC: Add riscv64 unknown linux musl support

[eshattow]

All three current versions of Alpine Linux (musl) have RISC-V support, though riscv6gc-unknown-linux-musl is listed by Rust foundation as Tier 2; riscv6gc-unknown-linux-gnu is however listed as Tier 2 (with Tools).

In support of rust-lang/docker-rust#272

[rustbot]

☔ The latest upstream changes (possibly #4849) made this pull request unmergeable. Please resolve the merge conflicts.

[rami3l]

@eshattow Hello, and sorry for the late reply 🙏

Are there any updates on this matter? Is this PR waiting for something to happen? I'd love to help if you need anything to land it. Many thanks in advance 🙇

[eshattow]

Thanks @rami3l no actionables yet, I have been looking at the powerpc promotion (to Tier 2) PR as an example. Welcome any advice or guidance :-) I have an amd64 desktop to run the build, also StarFive JH7110 (riscv64gc) and SpacemiT K3 (riscv64a23) boards to build or test with. Admittedly I don't know much about rust or the build system previously this is from my viewpoint something that needs done to move along the official rust docker images that much CI/CD depends on in the GitHub-using community; learning as I go.

One point I am unclear about is if we have RISE RISC-V runners or not configured for the GitHub account? Should I be writing this as a cross-compiled build with amd64 or instead natively with the ubuntu-24.04-riscv runner; or is it a blend of both for build and test?

[rami3l]

@eshattow Hello, thanks again for the update! I don't think we have riscv runners here at all, but in principle you won't need that to achieve Tier 2. So just adding a cross compilation target on Linux would be enough.

For a very recent example you can see #4908.