target/riscv: Add zcmop extension

target/riscv: Add zcmop extension

Zcmop defines eight 16-bit MOP instructions named C.MOP.n, where n is
an odd integer between 1 and 15, inclusive. C.MOP.n is encoded in
the reserved encoding space corresponding to C.LUI xn, 0.

Unlike the MOPs defined in the Zimop extension, the C.MOP.n instructions
are defined to not write any register.

In current implementation, C.MOP.n only has an check function, without any
other more behavior.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Message-ID: <20240709113652.1239-4-zhiwei_liu@linux.alibaba.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>