disas/riscv: Make rv_op_illegal a shared enum value

The enum value 'rv_op_illegal' does not represent an
instruction, but is a catch-all value in case we have
no match in the decoder. Let's make the value a shared
one, so that other compile units can reuse it.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Message-Id: <20230612111034.3955227-5-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>