Support for ARM Musl targets

[lilianmoraru]

Rust now supports these targets:

arm-unknown-linux-musleabi
arm-unknown-linux-musleabihf
armv7-unknown-linux-musleabihf

Could we get support for these targets?(Although, the packages are not yet uploaded 😄 )

[jonas-schievink]

Shouldn't support for them land automatically once they're uploaded?

[brson]

The actual work here needs to be done in rust-buildbot but there are some bugs there preventing it from happening.

[lilianmoraru]

Alex Crichton added support for it in rust-buildbot.

[lilianmoraru]

@jonas-schievink I don't know how exactly rustup works.
I tried now: rustup target add arm-unknown-linux-musleabi
And I get: error: component 'rust-std' for 'arm-unknown-linux-musleabi' is unavailable for download.

Although, I am not sure that arm-unknown-linux-musleabi was already built(I just guessed since 9 hours have passed).

[lilianmoraru]

Big thanks to @alexcrichton, these targets work now:

• arm-unknown-linux-musleabi
• arm-unknown-linux-musleabihf
• armv7-unknown-linux-musleabihf
• asmjs-unknown-emscripten
• wasm32-unknown-emscripten

num_cpus as a test, compiles with all the musl targets but not with emscripten.
Update: More complex applications that require linking other libraries fail.
Most probably need the cargo configs.