[llvm] [PowerPC] Fix use of FPSCR builtins in smmintrin.h (PR #67299)

[Stefan Pintilie via llvm-commits]

https://github.com/stefanp-ibm commented:

With respect to the comment from Nemanja I'm okay with this not being 100% the same as GCC. I'm sure we will get questions in the future but really we should discourage the use of `__builtin_mffsl` on pre-Power 9. 

https://github.com/llvm/llvm-project/pull/67299