Merge pull request #15236 from benpicco/cpu/cortexm_common_idle_flush

cpu/cortexm_common: flush pipeline before disabling interrupts in idle

cpu/cortexm_common/thread_arch.c

@@ -510,5 +510,6 @@ void sched_arch_idle(void)
     /* Briefly re-enable IRQs to allow pending interrupts to be serviced and
      * have them update the runqueue */
     __enable_irq();
+    __ISB();
     __disable_irq();
 }