[clang] [Clang][RISCV] Add missing support for `__builtin_riscv_cpop_32/64` (PR #76256)

[Craig Topper via cfe-commits]

topperc wrote:

Can we just use __builtin_popcount and __builtin_popcountll directly in the header file?

https://github.com/llvm/llvm-project/pull/76256