Allow toolchain rules to also wrap Arm Clang

[oliverlee]

@hexdae would you accept a set of PRs that integrate Arm Clang? I think the existing toolchain rules could be extended to allow the use to specify use of either Arm GNU or Arm Clang.

I have a proof-of-concept here but this would need some changes (which I have yet to think about).

Before adding any changes related to Arm Clang, it may be useful to merge these:
remove duplicated toolchain macros: oliverlee@2039a64
provide build file for user-specified toolchain files: oliverlee@9d576bc

It could also be useful to add tests for cross-compiled binaries via QEMU. I have those sorts of tests in cortex_m but that project requires Nix to be available and obtains the tool from Nixpkgs.

[hexdae]

I'm open to it, let's do it, I can merge the two outstanding PRs

[oliverlee]

Sorry, I haven't had a lot of free time recently. Hopefully I'll have a chance to work on this next month.

[oliverlee]

I haven't been able to prioritize this, but I haven't forgotten. I'm not sure when I'll be able to get around to it.