Add CI for ARM and make it pass

[kamalmarhubi]

This could be done similarly to libc using QEMU: https://github.com/rust-lang-nursery/libc/blob/master/ci/README.md#qemu

[posborne]

I have started working on this by taking the ci contents from libc and modifying to work with nix. Hopefully have a PR up soon -- I'll probably wrap in fixes in the PR as well so the build is green (seperate commits).

[kamalmarhubi]

@posborne that's awesome! Did you see rust-everywhere? I saw it go by on /r/rust yesterday. It sets up the ARM QEMU stuff on travis.

[posborne]

Hmm, I'll have to look at rust-everywhere a bit more -- Seems like the main qemu bit is here which is not really revelatory: https://github.com/japaric/rust-everywhere/blob/master/ci/script.sh#L24. Made it pretty far last night but bumped into some issues discovering tests: rust-lang/cargo#1924. I can probably hack around that in the short term.

With nix, we are using unittests in the source itself whereas libc has a separate binary that is executed that performs most testing. I think what we do with nix is reasonable, it just means that we need to do a little more work to figure out which cross-compiled executables need to be executed in various environments.

The alternative would be to setup a working rust/cargo in the target environment, but that would take quite a bit more time and isn't really worth it IMO.

[kamalmarhubi]

With nix, we are using unittests in the source itself whereas libc has a separate binary that is executed that performs most testing. I think what we do with nix is reasonable

Ah that's an interesting difference. I haven't looked too much into what libc does in CI, other than knowing that it's a little annoying to run the tests.

[posborne]

Complete! 🎉