[RISCV] Fix P-extension instruction names per spec 0.19 (#179961)
Fix instruction naming to match P-extension specification 0.19:
- pnsari.b -> pnsrari.b (Packed Narrowing Shift Right Arithmetic
Rounding)
- pnsari.h -> pnsrari.h
- nsari -> nsrari
- paax.dhx -> paas.dhx (Packed Average Add/Sub, not Add/Add-Cross)
The instruction encodings remain unchanged as they were already correct.
Ref: https://www.jhauser.us/RISCV/ext-P/RVP-baseInstrs-Sail-019.txt