[clang] [llvm] [RISCV][P-ext] Add packed multiply-parts intrinsics (PR #218875)

[via llvm-commits]

sihuan wrote:

`__riscv_mulu_w{00,01,11}` are spelled `_u64` here; P-ext-intrinsics.adoc says
`_i64` although they return `uint64_t` — the only entries in that section whose
suffix does not match the return type. riscv/riscv-p-spec#346 fixes the spec.


https://github.com/llvm/llvm-project/pull/218875