Update paths to homebrew LLVM for M1 macs

[LegNeato]

/opt/homebrew/ is used by hombrew on Apple Silicon Macs, but /usr/local/ is still used on Intel Macs, and we want to support both. This lists both paths in the README.md, clarifying which is for Intel vs. Apple Silicon Macs.

These paths are also hardcoded in c2rust-build-paths/src/lib.rs and c2rust-analyze/tests/filecheck.rs. The latter is meant to be consolidated under c2rust-build-paths, but we'll keep them as is for now. We need to also update these to include the Apple Silicon homebrew paths.

Now, c2rust builds fine on Intel Macs (CI) and Apple Silicon Macs (@LegNeato's M1).

[kkysen]

Is this the M1 path? Maybe we should just list both paths, and in c2rust-ast-exporter, too, where it looks in a bunch of paths for llvm-config.

[kkysen]

/opt/homebrew/ is used by hombrew on M1 macs, but /usr/local/ is still used on Intel macs, and we want to support both. So we should list both paths in the README.md, and potentially also say one is for Intel macs and one is for M1 macs (M2, etc., too).

These paths are also hardcoded in c2rust-build-paths/src/lib.rs and c2rust-analyze/tests/filecheck.rs. The latter is meant to be consolidated under c2rust-build-paths. We need to also update these to include the M1 homebrew paths.

[kkysen]

We'll probably want to eventually generate all these different but similar paths to search, or search by glob/regex or something, but for now, this looks great and updates things in all the right places I believe.

[kkysen]

By the way, @fw-immunant, I think we should merge this before publishing a new c2rust version.

[fw-immunant]

By the way, @fw-immunant, I think we should merge this before publishing a new c2rust version.

I agree. This looks good to me, but I'm not familiar with Homebrew/macOS. Should we wait for @thedataking to review, or just go ahead and merge?

[kkysen]

@thedataking doesn't have an M1, right? We can wait for him to take a quick look, but I'm not sure he'll have any extra insight. I think if it works on both on @LegNeato's M1 mac and @thedataking's Intel mac it should be good.

[thedataking]

LGTM. Note that the CI workflow already tests Intel Macs.

nit: since this patch also fixes capitalization, it should use ARM, not Arm, since it is an acronym. We could also just say Intel Macs and Apple Silicon Macs. No big deal though.