[clang] [llvm] [AArch64] FEAT_SPEv1p2 is optional in v8.7-A and v9.2-A (PR #123336)

[David Green via llvm-commits]

davemgreen wrote:

Is this a system-reg only extension?
It was enabled in #115296, which has an explanation why it was enabled.

I'm not sure how well we implement the sys-reg only extensions always being enabled idea, or if the best way to handle that is making them required features. But this seemed intentional.

https://github.com/llvm/llvm-project/pull/123336