disas/riscv: Move rv_op_illegal to riscv.c

illegal is no longer used in other files.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20260812223142.349142-45-richard.henderson@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>