[clang] [Clang] Bring initFeatureMap back to AArch64TargetInfo. (PR #96832)

[William G Hatch via cfe-commits]

willghatch wrote:

@labrinea please take a look.  The [PR mentioned](https://github.com/llvm/llvm-project/pull/94279) above removed this code and broke AArch64 feature detection for us.

https://github.com/llvm/llvm-project/pull/96832