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>

include/opcode/riscv.h

@@ -558,10 +558,9 @@ enum
   M_CALL,
   M_J,
   M_LI,
-  M_ZEXTH,
+  M_EXTH,
   M_ZEXTW,
   M_SEXTB,
-  M_SEXTH,
   M_VMSGE,
   M_NUM_MACROS
 };