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

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic approved this pull request.

Please make sure the commit message reflects the extended change.  Otherwise LGTM, but maybe give a couple days if anyone has a second opinion.

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