[clang] [llvm] [AArch64] Add optional extensions enabled on Grace (PR #127620)

[Sjoerd Meijer via cfe-commits]

https://github.com/sjoerdmeijer commented:

It would be good to mention in the description:
-  that Grace is no longer an alias, but is a separate CPU definition. 
- which optional extensions are now enabled.

https://github.com/llvm/llvm-project/pull/127620