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: https://github.com/oliverlee/bazel-arm-none-eabi/commit/2039a64d25db5400e202ac302c98450f95c2feb7 provide build file for user-specified toolchain files: https://github.com/oliverlee/bazel-arm-none-eabi/commit/9d576bc179b564ed7f0a02985f0562e50e8e6da8

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.