Auto merge of #44779 - tjkirch:master, r=alexcrichton

Auto merge of #44779 - tjkirch:master, r=alexcrichton

Add aarch64-unknown-linux-musl target

This adds support for the aarch64-unknown-linux-musl target in the build and CI systems.

This addresses half of issue #42520.

The new file `aarch64_unknown_linux_musl.rs` is a copy of `aarch64_unknown_linux_gnu.rs` with "gnu" replaced by "musl", and the added logic in `build-arm-musl.sh` is similarly a near-copy of the arches around it, so overall the changes were straightforward.

Testing:

```
$ sudo ./src/ci/docker/run.sh cross
...
Dist std stage2 (x86_64-unknown-linux-gnu -> aarch64-unknown-linux-musl)
Building stage2 test artifacts (x86_64-unknown-linux-gnu -> aarch64-unknown-linux-musl)
   Compiling getopts v0.2.14
   Compiling term v0.0.0 (file:///checkout/src/libterm)
   Compiling test v0.0.0 (file:///checkout/src/libtest)
    Finished release [optimized] target(s) in 16.91 secs
Copying stage2 test from stage2 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / aarch64-unknown-linux-musl)
...
Build completed successfully in 0:55:22
```

```
$ rustup toolchain link local obj/build/x86_64-unknown-linux-gnu/stage2
$ rustup default local
```

After setting the local toolchain as default, and adding this in ~/.cargo/config:

```
[target.aarch64-unknown-linux-musl]
linker = "aarch64-linux-musl-gcc"
```

...then the toolchain was able to build a working ripgrep as a test:

```
$ readelf -a target/aarch64-unknown-linux-musl/debug/rg | grep -i interpreter
$ readelf -a target/aarch64-unknown-linux-musl/debug/rg | grep NEEDED
$ file target/aarch64-unknown-linux-musl/debug/rg
target/aarch64-unknown-linux-musl/debug/rg: ELF 64-bit LSB executable, ARM aarch64, version 1 (GNU/Linux), statically linked, BuildID[sha1]=be11036b0988fac5dccc9f6487eb780b05186582, not stripped
```