Add aarch64-unknown-linux-musl support

[Gelbpunkt]

See #2003

I have no clue what to do with the .github/workflows files as they state not to edit them. Same applies to tests/channel-rust-nightly-example2.toml which seems autogenerated.

This is mainly the result of a grep for aarch64-unknown-linux-gnu and appropiate changes

[kinnison]

You need to fix up the stuff I mentioned, and then in a separate commit, please include the results of running bash ci/actions-templates/gen-workflows.sh

[Gelbpunkt]

@kinnison all done!

[kinnison]

Okay the build has failed - the compiler appears to be missing - https://github.com/rust-lang/rustup/pull/2493/checks?check_run_id=1134461439

This does not bode well.

[kinnison]

This time the build got a lot further but then failed to link because of the floating point library perhaps? I am not a musl expert. Look at https://github.com/rust-lang/rustup/pull/2493/checks?check_run_id=1134498637 and see if you have any ideas.

[Gelbpunkt]

This time the build got a lot further but then failed to link because of the floating point library perhaps? I am not a musl expert. Look at https://github.com/rust-lang/rustup/pull/2493/checks?check_run_id=1134498637 and see if you have any ideas.

Seems like rust-lang/rust#46651 - what would be the steps to set compile arguments in the build here? (Can probably imagine using RUSTFLAGS)

[kinnison]

I am not sure as I've never had to do that before. I think you'll need to see help from outside, but I'd guess setting the ENV in the dockerfile maybe?

[Gelbpunkt]

ENV would probably work. Just a question - this seems to run on a stable rust toolchain base, which does not include rust-lang/compiler-builtins#377 (which should fix this). Is it possible to use nightly or are rustflags in ENV fine for now?

[kinnison]

We explicitly use stable. If it makes more sense, we can wait for a release which includes the fix, rather than pushing for things now.

[Gelbpunkt]

It seems to work with the rustflags. I don't think it hurts to include them and it is easy to remove once 1.48 is stable and probably better than waiting.

[kinnison]

Yep this looks plausible. This needs rebasing into a commit which adds support for the platform, then another which contains the regenerated workflows. This platform should probably be skip-pr skip-master for now.

[Gelbpunkt]

@kinnison I hope this is what you expected 😄 If not, let me know and I can change it

[kinnison]

Looks good, thank you. We'll see how it goes at the next release :D