[compiler-rt] [SME][compiler-rt] Guard .variant_pcs under HAS_ASM_SME macro. (PR #68991)

[Sander de Smalen via llvm-commits]

sdesmalen-arm wrote:

> If `.variant_pcs` is optional, why do we need it at all?

AIUI `.variant_pcs` is not optional. I've removed the support to build these routines when SME ASM support is not available, which also gets rid of some of the macros and cleans up the code a bit further.

https://github.com/llvm/llvm-project/pull/68991