[clang] [RISCV] Add riscv_simd.h for P extension intrinsics (PR #181115)

[Alexander Richardson via cfe-commits]

https://github.com/arichardson commented:

Not sure if this suggestion makes sense, but since all the content is super regular I was wondering if it makes sense to use a macro (or have tablegen generate it) to avoid accidental copy-paste typos when we add more operations in the future.

https://github.com/llvm/llvm-project/pull/181115