Test rustup on cross-compile targets

[brson]

People depend on rustup for all sorts of hardware that I don't have, and that isn't tested. It would be nice to verify that it actually works. Best to come up with some solution that works in a Docker container and can run cargo test against an emulator, probably qemu.

cc #687

@japaric Do you have a solution for running tests against emulators for common targets?

[japaric]

Yeah, I've been using this docker image (Dockerfile) to test rustc-builtins, a Rust port of compiler-rt builtins, on almost (*) every single arch that Rust supports via QEMU emulation.

I've been meaning to test integrating the approach used in that repo into the rust-lang/libc repo but I've been busy lately. I should have time to look into it soon though. If I have extra time, I can look into integrating that into this repository as well.

(*) Can't test on ppc64le because QEMU for that arch appears to be broken; it segfaults even running this simple C program: int main() { return 0; }.

[japaric]

Oh, and I don't recommend using that docker image to build binary releases. The image uses Ubuntu 16.04 and its cross toolchains come with glibc-2.2x. Using those toolchains would generate a binary that depends on a too new glibc version, which would make the binaries less portable.