[clang] [AArch64][Clang] Add support for __arm_agnostic("sme_za_state") (PR #121788)

[Kerry McLaughlin via cfe-commits]

kmclaughlin-arm wrote:

Please can you also extend `mangleSMEAttrs` in ItaniumMangle.cpp to include the `__arm_agnostic` attribute?

https://github.com/llvm/llvm-project/pull/121788