cpu/cortexm_common: flush pipeline after PendSV

https://interrupt.memfault.com/blog/arm-cortex-m-exceptions-and-nvic#pendsv-example

cpu/cortexm_common/thread_arch.c

@@ -297,6 +297,9 @@ void thread_yield_higher(void)
     /* trigger the PENDSV interrupt to run scheduler and schedule new thread if
      * applicable */
     SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
+    /* flush the pipeline. Otherwise we risk that subsequent instructions are
+     * executed before the IRQ has actually triggered */
+    __ISB();
 }
 
 void __attribute__((naked)) __attribute__((used)) isr_pendsv(void) {