RISC-V: reduce redundancy in sign/zero extension macro insn handling

RISC-V: reduce redundancy in sign/zero extension macro insn handling

Fold M_{S,Z}EXTH, deriving signed-ness from the incoming mnemonic. Fold
riscv_ext()'s calls md_assemblef(), the first of which were entirely
identical, while the other pair differed in just a single character.

Acked-by: Palmer Dabbelt <palmer@rivosinc.com>