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

[Jon Roelofs via cfe-commits]

jroelofs wrote:

Correct, and that's fine: nothing we've previously shipped supports FEAT_CSSC.

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