[llvm] [AArch64] support `.arch_extension` for features that the CLI also accepts (PR #169999)

[David Spickett via llvm-commits]

DavidSpickett wrote:

Thanks! I've updated the branch and enabled the auto-merge so if CI is green (chances are it will be), it will be merged.

https://github.com/llvm/llvm-project/pull/169999