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>

target/riscv/cpu.h

@@ -331,6 +331,7 @@ struct CPUArchState {
     struct CPUWatchpoint *cpu_watchpoint[RV_MAX_TRIGGERS];
     QEMUTimer *itrigger_timer[RV_MAX_TRIGGERS];
     int64_t last_icount;
+    bool itrigger_enabled;
 
     /* machine specific rdtime callback */
     uint64_t (*rdtime_fn)(void *);