Add aarch64-unknown-linux-musl host builds

[Gelbpunkt]

This adds aarch64-unknown-linux-musl to the hosts list and adds the build to the dist-arm-linux builder as @Mark-Simulacrum suggested to me in Zulip. @jyn514 requested to be mentioned 😄

I had to update the config for crosstool-ng as it had a prompt about the glibc version.

I ran src/ci/docker/run.sh dist-arm-linux to test it.

Build completed successfully in 1:31:50
Compile requests              8180
Compile requests executed     8135
Cache hits                     287
Cache misses                  7848
Cache timeouts                   0
Cache read errors                0
Forced recaches                  0
Cache write errors               0
Compilation failures             0
Cache errors                     0
Non-cacheable compilations       0
Non-cacheable calls             36
Non-compilation calls            9
Unsupported compiler calls       0
Average cache write          0.000 s
Average cache read miss      6.389 s
Average cache read hit       0.000 s
Cache location             Local disk: "/sccache"
Cache size                     202 MiB
Max cache size                  10 GiB
== clock drift check ==
  local time: Sun Sep  6 19:30:17 UTC 2020
  network time: Sun, 06 Sep 2020 19:30:17 GMT
== end clock drift check ==

Only errors were in miri due to struct fields being private (already been reported here)

Edit: Maybe it is helpful if I add that it is a working compiler

/rust-nightly-aarch64-unknown-linux-musl # ash install.sh 
install: creating uninstall script at /usr/local/lib/rustlib/uninstall.sh
install: installing component 'rustc'
install: installing component 'cargo'
install: installing component 'rls-preview'
install: installing component 'rust-analyzer-preview'
install: installing component 'clippy-preview'
install: installing component 'rustfmt-preview'
install: installing component 'llvm-tools-preview'
install: installing component 'rust-analysis-aarch64-unknown-linux-musl'
install: installing component 'rust-std-aarch64-unknown-linux-musl'
install: WARNING: failed to run ldconfig. this may happen when not installing as root. run with --verbose to see the error

    Rust is ready to roll.

/ # cat test.rs 
fn main() { println!("hello world"); }
/ # rustc test.rs
/ # ./test
hello world
 # file test
test: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, not stripped

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pietroalbini (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[ehuss]

Is this different from #75751?

[pietroalbini]

Thanks for opening the PR!

Indeed, #75751 is already open with the same changes. Closing this in favor of the other one.

[alex]

Duplicating my comment from #75751 -- I think this PR is more complete than mine and it'd be better to review it instead of mine.

[Gelbpunkt]

@alex Hi, I am sorry for the late reply and for opening this as I did not see your PR earlier when I made this one. While I don't completely grasp how your PR would be less complete, I've updated this one based on missing parts in it with your changes. Thanks a lot for your effort!

[alex]

@Gelbpunkt you got your PR all the way to fully building, if I understand correctly, and mine wasn't :-)

[jyn514]

Copying from #75751 (comment):

This was discussed as part of the compiler weekly meeting

More specifically, I think the T-compiler team was generally in favor of landing this.

However it still needs a decision from T-infra I think.

[pietroalbini]

The code change looks fine, and we'll discuss in this week's infra team meeting.

[pietroalbini]

Discussed this in the infrastructure team meeting, and we agreed to approve landing this! Thanks for sending the PR!

@bors r+ rollup=never

[bors]

📌 Commit bfb56d8 has been approved by pietroalbini

[bors]

⌛ Testing commit bfb56d8 with merge ff806b8...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: pietroalbini
Pushing ff806b8 to master...