cpu/cortexm_common: replace irq_restore by __set_PRIMASK for stm32l152re

- The __NOP() that was added in #8518 is now remooved.
- When DBG_STANDBY, DBG_STOP or DBG_SLEEP are set in DBG_CR a hardfault
  occurs on wakeup from sleep. This was first diagnosed in #8518. When
  enabled, a hardfault occured when returning from a branch to irq_restore()
  we avoid the call by inlining the function call. See #xxxxx for more
  details.

cpu/cortexm_common/include/cpu.h

@@ -165,16 +165,15 @@ static inline void cortexm_sleep(int deep)
     else {
         SCB->SCR &= ~(SCB_SCR_SLEEPDEEP_Msk);
     }
-
     /* ensure that all memory accesses have completed and trigger sleeping */
     unsigned state = irq_disable();
     __DSB();
     __WFI();
 #if defined(CPU_MODEL_STM32L152RE)
-    /* STM32L152RE crashes without this __NOP(). See #8518. */
-    __NOP();
-#endif
+    __set_PRIMASK(state);
+#else
     irq_restore(state);
+#endif
 }
 
 /**