[llvm] [AArch64][GlobalISel] Implement selectVaStartAAPCS (PR #106979)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Can we put a status page (AArch64.rst) under https://github.com/llvm/llvm-project/tree/main/llvm/docs/GlobalISel?
https://discourse.llvm.org/t/enabling-globalisel-for-apple-aarch64-platforms/63953/7
Wait for it, SVE is coming.
I leave status pages for other targets to other PRs.

https://github.com/llvm/llvm-project/pull/106979