[llvm] [llvm][AArch64] SVE2 is an optional feature in ARMv9.0a (PR #96007)

[Jon Roelofs via llvm-commits]

jroelofs wrote:

This is a baby step toward fixing `apple-m4`'s version: v8.7 => v9.2

https://github.com/llvm/llvm-project/pull/96007