runtime: use waitForEvents when appropriate

This is better than using the wfe or wfi instruction directly as it can
be replaced with build tags.

src/runtime/runtime_atsamd21.go

@@ -308,7 +308,7 @@ func timerSleep(ticks uint32) bool {
 	sam.RTC_MODE0.INTENSET.SetBits(sam.RTC_MODE0_INTENSET_CMP0)
 
 wait:
-	arm.Asm("wfe")
+	waitForEvents()
 	if timerWakeup.Get() != 0 {
 		return true
 	}

src/runtime/runtime_atsamd51.go

@@ -292,7 +292,7 @@ func timerSleep(ticks uint32) bool {
 	sam.RTC_MODE0.INTENSET.SetBits(sam.RTC_MODE0_INTENSET_CMP0)
 
 wait:
-	arm.Asm("wfe")
+	waitForEvents()
 	if timerWakeup.Get() != 0 {
 		return true
 	}

src/runtime/runtime_nrf.go

@@ -3,7 +3,6 @@
 package runtime
 
 import (
-	"device/arm"
 	"device/nrf"
 	"machine"
 	"runtime/interrupt"
@@ -114,6 +113,6 @@ func rtc_sleep(ticks uint32) {
 	}
 	nrf.RTC1.CC[0].Set((nrf.RTC1.COUNTER.Get() + ticks) & 0x00ffffff)
 	for rtc_wakeup.Get() == 0 {
-		arm.Asm("wfi")
+		waitForEvents()
 	}
 }