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

[weiwei chen via cfe-commits]

weiweichen wrote:

> Could you explain more about what broke? Are you using target(..) attributes?

`TargetInfo::CreateTargetInfo` is giving up back empty `opt->featureMap` for `AArch64`. But before the [PR mentioned](https://github.com/llvm/llvm-project/pull/94279), we can get a list of features. We can also get a list of feature for other targets like `X86`, etc. Is there a different way to get them back for AArch64 if this change is not desirable?

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