[clang] [llvm] [RISCV] Implement Clang Builtins for XAndesPerf Extension (PR #147018)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

Should we document this in https://github.com/riscv-non-isa/riscv-c-api-doc/blob/main/src/c-api.adoc?

https://github.com/llvm/llvm-project/pull/147018