target/riscv: Merge argument sets for insn32 and insn16

target/riscv: Merge argument sets for insn32 and insn16

In some cases this allows us to directly use the insn32
translator function.  In some cases we still need a shim.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>