all: change special type __volatile to pragma //go:volatile

This is one step towards removing unnecessary special casts in most
cases. It is also part of removing as much magic as possible from the
compiler (the pragma is explicit, the special name is not).

src/runtime/runtime_nrf.go

@@ -81,9 +81,10 @@ func ticks() timeUnit {
 	return timestamp
 }
 
-type __volatile bool
+//go:volatile
+type isrFlag bool
 
-var rtc_wakeup __volatile
+var rtc_wakeup isrFlag
 
 func rtc_sleep(ticks uint32) {
 	nrf.RTC0.INTENSET = nrf.RTC_INTENSET_COMPARE0