cpu/cortexm_common: add NOP after WFI to avoid hardfault on stm32l152

cpu/cortexm_common/include/cpu.h

@@ -107,6 +107,10 @@ static inline void cortexm_sleep(int deep)
     unsigned state = irq_disable();
     __DSB();
     __WFI();
+#if defined(CPU_MODEL_STM32L152RE)
+    /* STM32L152RE crashes without this __NOP(). See #8518. */
+    __NOP();
+#endif
     irq_restore(state);
 }