new binary releases for mips and arm

[japaric]

Just a reminder of the extra binaries that can be build in the linux-cross image after its next rebuild.

• libstd for mips(el)-unknown-linux-musl (2 targets)
  • Image: linux-cross
  • Uses: the old build system
  • Probably just tack thxs on top of the 'nightly-dist-rustc-cross-linux' builder. Add both targets
    to the --target argument of the configure script.
• rustc for arm(v7)-unknown-linux-gnueabi(hf) (3 hosts)
  • Image: linux-cross
  • Uses: rustbuild
  • Blocked on: rustbuild: Implement make dist rust-lang/rust#32237
  • This wants a new builder.
  • Since this job will also produce libstd tarballs, perhaps we can remove these 3 targets from the 'nightly-dist-rustc-cross-linux' builder and have this builder upload 3 rustc tarballs and 3 libstd tarballs.
• libstd for i686-unknown-linux-musl
  • Image: linux
  • Uses: the old build system
  • Blocked on linux: build musl/libunwind for i686 #79
  • This should go in its own builder because AFAICT one can't build libstd for two static musl targets. And we can't do that because we would need to pass one --musl-root to configure for each target but that flag only accepts one argument.

cc @alexcrichton

[japaric]

Added libstd for i686-musl to the top comment.

[alexcrichton]

Working on adding netbsd/freebsd compilers as well, hope to have this by the end of the week.

[alexcrichton]

Lotsa buildbot changes here as well to take care of

[alexcrichton]

Extracting out the last bit to #88