[llvm] [AArch64][TargetParser] autogen ArchExtKind enum (PR #90314)

[Jonathan Thackray via llvm-commits]

https://github.com/jthackray commented:

LGTM, a good cleanup of duplicated code, although we will lose the comments where the names don't match, such as AEK_PREDRES // FEAT_SPECRES, AEK_PERFMON // FEAT_PMUv3. I guess we can note them elsewhere.

https://github.com/llvm/llvm-project/pull/90314