[clang] [Clang][AArch64] set default mtune (PR #179136)

[Ricardo Jesus via cfe-commits]

https://github.com/rj-jesus approved this pull request.

I don't know what the default tuning for Apple Silicon should be, but the remaining changes look good to me (I just left a few suggestions).

Please give it a day or so before merging in case anyone has other comments, and maybe also update the PR title to say that this patch primarily affects macOS.

https://github.com/llvm/llvm-project/pull/179136