Auto merge of #76420 - Gelbpunkt:aarch64-linux-musl, r=pietroalbini

Add aarch64-unknown-linux-musl host builds

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 :smile:

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](https://github.com/rust-lang/rust/issues/76337))

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

0 files changed, 0 insertions, 0 deletions