Make the ci infrastructure reusable from other crates

[gnzlbg]

I'd like to reuse most / all of the ci infrastructure from the stdsimd crate, so I'd like to ask if it would be possible to refactor this in such a way that this becomes possible.

[malbarbo]

@gnzlbg I believe that cross and trust already provides reusable ci infrastructure. I known you are interested in running the foreign binaries in qemu-system because of cross-rs/cross#146.

I'm trying to add a qemu-system runner in cross, so I suppose we can discuss this issue there.

[gnzlbg]

@malbarbo awesome, I'll subscribe there :)