Use the wait-for-event instruction to put the core we have just enabled

to sleep while it waits to start scheduling. The boot core can then use
the send-event instruction to wake the cores when they should enter the
scheduler.

MFC after:	1 week

sys/arm/arm/mp_machdep.c

@@ -185,8 +185,11 @@ init_secondary(int cpu)
 	atomic_add_rel_32(&mp_naps, 1);
 
 	/* Spin until the BSP releases the APs */
-	while (!aps_ready)
-		;
+	while (!atomic_load_acq_int(&aps_ready)) {
+#if __ARM_ARCH >= 7
+		__asm __volatile("wfe");
+#endif
+	}
 
 	/* Initialize curthread */
 	KASSERT(PCPU_GET(idlethread) != NULL, ("no idle thread"));
@@ -353,6 +356,10 @@ release_aps(void *dummy __unused)
 		arm_unmask_irq(i);
 	}
 	atomic_store_rel_int(&aps_ready, 1);
+	/* Wake the other threads up */
+#if __ARM_ARCH >= 7
+	armv7_sev();
+#endif
 
 	printf("Release APs\n");