runtime: update to avoid //go:volatile

There was exactly one change in the output of the smoke tests:
examples/test. However, it still runs just fine on a PCA10040.

src/runtime/runtime_nrf.go

@@ -6,6 +6,7 @@ import (
 	"device/arm"
 	"device/nrf"
 	"machine"
+	"runtime/volatile"
 )
 
 type timeUnit int64
@@ -79,14 +80,11 @@ func ticks() timeUnit {
 	return timestamp
 }
 
-//go:volatile
-type isrFlag bool
-
-var rtc_wakeup isrFlag
+var rtc_wakeup volatile.Register8
 
 func rtc_sleep(ticks uint32) {
 	nrf.RTC1.INTENSET.Set(nrf.RTC_INTENSET_COMPARE0)
-	rtc_wakeup = false
+	rtc_wakeup.Set(0)
 	if ticks == 1 {
 		// Race condition (even in hardware) at ticks == 1.
 		// TODO: fix this in a better way by detecting it, like the manual
@@ -94,7 +92,7 @@ func rtc_sleep(ticks uint32) {
 		ticks = 2
 	}
 	nrf.RTC1.CC[0].Set((nrf.RTC1.COUNTER.Get() + ticks) & 0x00ffffff)
-	for !rtc_wakeup {
+	for rtc_wakeup.Get() == 0 {
 		arm.Asm("wfi")
 	}
 }
@@ -103,5 +101,5 @@ func rtc_sleep(ticks uint32) {
 func handleRTC1() {
 	nrf.RTC1.INTENCLR.Set(nrf.RTC_INTENSET_COMPARE0)
 	nrf.RTC1.EVENTS_COMPARE[0].Set(0)
-	rtc_wakeup = true
+	rtc_wakeup.Set(1)
 }