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 😄
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
```
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