[llvm] [clang] [clang][AArch64] Add a -mbranch-protection option to enable GCS (PR #75486)

[Jonathan Thackray via llvm-commits]

https://github.com/jthackray approved this pull request.

I'm not an expert in this area, but this code LGTM.

https://github.com/llvm/llvm-project/pull/75486