Add musl targets to CI.

[cbiffle]

I've added musl-gcc here, and not musl-clang, because I can't figure out
what the package name for musl-clang is on Ubuntu.

I agree to license my contributions to each file under the terms given
at the top of each file I changed.

[briansmith]

@cbiffle Your PR is still marked as a draft. Is it ready to be reviewed?

[briansmith]

OK, I looked at this and it seems OK for now. However, I would really appreciate it if somebody could get it working with clang as the toolchain instead of musl-gcc. At least the last time I tried it, GCC didn't do a good job optimizing some of the multi-precision arithmetic. My understanding is that to build with clang, we "just" need to use TARGET_CC=clang

$ sudo apt remove musl musl-dev musl-tools
$ sudo apt install musl-dev
$ TARGET_CC=clang cargo test --target=x86_64-unknown-linux-musl

Actually, for me it worked even without the sudo apt install musl-dev step.

[briansmith]

I've added musl-gcc here, and not musl-clang, because I can't figure out
what the package name for musl-clang is on Ubuntu.

My understanding is there is no "musl-clang". Instead, clang always works as a cross compiler, so installing any(?) version of clang will install one that can target musl. Even this ancient one was able to do it, as this is the version I used in my previous comment:

$ clang --version
clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[briansmith]

Given the aforementioned comments and also the fact that I'm trying to remove the master branch today, I'm closing this MR. I would love to see a new, simpler, version of this that uses clang instead of GCC.

[briansmith]

Also, @cbiffle, thanks for your work on this!