[RISCV][P-ext] Add scalar mulq(r) intrinsics (#232316)

Add the scalar "Q-format" Multiplication intrinsics:
https://github.com/riscv/riscv-p-spec/blob/master/P-ext-intrinsics.adoc#q-format-multiplication

- `__riscv_mulq_i32`
- `__riscv_mulqr_i32`

RV32 selects `mulq`/`mulqr` directly. RV64 has no scalar forms, so the
intrinsics use `pmulq.w`/`pmulqr.w` on the low words, the same way as
the scalar `aadd`/`asub` intrinsics (#230710).

GitOrigin-RevId: c047b9cfbd1b08da1d9f6b8937380913fa68c11b