[clang] [llvm] [RISCV] Add Clang builtins for XCVbitmanip (PR #202564)

[Craig Topper via cfe-commits]

topperc wrote:

I don't think builtins should be the programmer interface. We usually use a header file to expose names like `__riscv_cv_....` to programmer's. The builtins are just used to translate between the header file and the compiler. The header file can make use of existing builtins for popcnt, rotate, and ff1.

https://github.com/llvm/llvm-project/pull/202564