[clang] [llvm] [clang][RISCV] Use macro to check if intrinsics are supported (PR #187197)

[Sam Elliott via llvm-commits]

lenary wrote:

Do we want a difference between `__riscv_v_intrinsic_*` and `__riscv_intrinsic_*`? 

https://github.com/llvm/llvm-project/pull/187197