[clang] [llvm] [RISCV] Preliminary P-ext intrinsics support (PR #174068)

[Kito Cheng via cfe-commits]

kito-cheng wrote:

For such simple operation I would prefer just use operator to implement on header, e.g.
```c
static __inline__
int8x4_t __attribute__((__always_inline__, __nodebug__)) __riscv_padd_i8x4(int8x4_t rs1, int8x4_t rs2)
{
  return rs1 + rs2;
}
```

https://github.com/llvm/llvm-project/pull/174068