[compiler-rt] [Compiler-rt] Add AArch64 routines for __arm_agnostic("sme_za_state") (PR #120059)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm approved this pull request.

A couple more recommendations but otherwise this looks good to me.

There's still some remove for improvement not least regarding `__arm_sme_state_size` where we go to pains to ensure the returned value is the minimum necessary.

I wonder if we need to be this strict because we could for example always return the current maximum when SME is available regardless of the state of the lazy save or SME2 availability. With that said, I've no evidence these routines have performance concerns so am happy to see how thing splay out.

https://github.com/llvm/llvm-project/pull/120059