[clang] [llvm] [RISCV] Add `zihintpause` LLVM/Clang intrinsic (PR #139519)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp commented:

Do we need a header for it (just like others in https://github.com/riscv-non-isa/riscv-c-api-doc/blob/main/src/c-api.adoc#intrinsic-functions)?

https://github.com/llvm/llvm-project/pull/139519