[llvm] [AArch64] support `.arch_extension dit` (PR #169999)

[Jonathan Thackray via llvm-commits]

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

Thanks, LGTM.

(Completely agree with the previous comment, that we should unify code so there's a single place to define arch extensions, so this doesn't reoccur)

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