Rustup: Promote x86_64-unknown-linux-musl from target to host toolchain

[mcandre]

Could the musl toolchain be promoted from something people access with

rustup target add x86_64-unknown-linux-musl

to something that people can

curl https://sh.rustup.rs -sSf | sh -s -- --default-host x86_64-unknown-linux-musl -y

?

[mbrubeck]

Depends on rust-lang/rust#40113.

[SimonSapin]

https://github.com/rust-lang/rust/pull/40113 has landed.

[dwijnand]

From @alexcrichton's comment https://github.com/rust-lang/rustup.rs/pull/1517#issuecomment-431007160:

I think it's probably not worth doing this until we get a musl host rustc working, otherwise this rustup won't have anything to download!

This is now tracked upstream as https://github.com/rust-lang/rust/issues/57439. So this ticket is (effectively) blocked by that.

[ebkalderon]

@dwijnand Looks like that upstream issue has been successfully resolved. Is this task ready to move forward?