wrctl instruction on nios2 target will cause checking cpu
interrupt but tcg_handle_interrupt() will call cpu_abort()
if the CPU gets an interrupt while it's not in 'can do IO'
state, so add gen_io_start around wrctl instruction. Also
at the same time, end the onging TB with DISAS_UPDATE.

Signed-off-by: Wentong Wu <wentong.wu@intel.com>
Message-id: 20200710233433.19729-3-wentong.wu@intel.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>