target/riscv: Add itrigger_enabled field to CPURISCVState

target/riscv: Add itrigger_enabled field to CPURISCVState

Avoid calling riscv_itrigger_enabled() when calculate the tbflags.
As the itrigger enable status can only be changed when write
tdata1, migration load or itrigger fire, update env->itrigger_enabled
at these places.

Signed-off-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221013062946.7530-5-zhiwei_liu@linux.alibaba.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>