cortexm_common: Clear PendSV request after idle sleep

The PendSV interrupt is used to request a scheduling operation. An
interrupt during the idle sleep can re-request the PendSV interrupt,
while the PendSV is still busy scheduling the next thread. This clears
the request after sleep to prevent triggering an extra PendSV interrupt
after the current PendSV handler finished.

cpu/cortexm_common/thread_arch.c

@@ -474,4 +474,5 @@ void sched_arch_idle(void)
 #endif
     irq_restore(state);
     NVIC_SetPriority(PendSV_IRQn, CPU_CORTEXM_PENDSV_IRQ_PRIO);
+    SCB->ICSR = SCB_ICSR_PENDSVCLR_Msk;
 }