[llvm] [LLVM] Update AArch64 maintainers (PR #120440)

[Amara Emerson via llvm-commits]

aemerson wrote:

Agreed we should have some more maintainers. I think having someone(s) from ARM would make sense. I don't know if the following people would be interested/have time but given how broad the parts of the backend are it would be good to have multiple people cover it:
 * @davemgreen for general AArch64 codegen
 * @sdesmalen-arm for the scalable vectorization/SVE/SME parts

I will also volunteer myself for AArch64's GlobalISel implementation and general backend

https://github.com/llvm/llvm-project/pull/120440