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

[Craig Topper via cfe-commits]

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+/*===---- riscv_simd.h - RISC-V P intrinsics -----------------===
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topperc wrote:

This line should be 80 characters long

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