[clang] [compiler-rt] [llvm] [FMV][AArch64] Add feature CSSC and detect on linux platform. (PR #132727)

[Jon Roelofs via llvm-commits]

jroelofs wrote:

> Hi @jroelofs @DanielKristofKiss. I am not sure how to detect on other platforms. Suggestions for apple and windows welcome.

The place to look for feature tests on Apple platforms is here: https://github.com/apple-oss-distributions/xnu/blob/main/osfmk/arm/cpu_capabilities_public.h.

https://github.com/llvm/llvm-project/pull/132727