target/riscv: Fix itrigger when icount is used

target/riscv: Fix itrigger when icount is used

When I boot a ubuntu image, QEMU output a "Bad icount read" message and exit.
The reason is that when execute helper_mret or helper_sret, it will
cause a call to icount_get_raw_locked (), which needs set can_do_io flag
on cpustate.

Thus we setting this flag when execute these two instructions.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230324064011.976-1-zhiwei_liu@linux.alibaba.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>