[llvm] [llvm][AArch64][TargetParser][NFC] Use parseArchExtension in parseModifier. (PR #80427)

[David Spickett via llvm-commits]

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

LGTM, glad to see more of this code getting centralised.

https://github.com/llvm/llvm-project/pull/80427