all: add compiler support for interrupts

This commit lets the compiler know about interrupts and allows
optimizations to be performed based on that: interrupts are eliminated
when they appear to be unused in a program. This is done with a new
pseudo-call (runtime/interrupt.New) that is treated specially by the
compiler.

src/runtime/runtime_nrf.go

@@ -6,6 +6,7 @@ import (
 	"device/arm"
 	"device/nrf"
 	"machine"
+	"runtime/interrupt"
 	"runtime/volatile"
 )
 
@@ -43,8 +44,13 @@ func initLFCLK() {
 
 func initRTC() {
 	nrf.RTC1.TASKS_START.Set(1)
-	arm.SetPriority(nrf.IRQ_RTC1, 0xc0) // low priority
-	arm.EnableIRQ(nrf.IRQ_RTC1)
+	intr := interrupt.New(nrf.IRQ_RTC1, func(intr interrupt.Interrupt) {
+		nrf.RTC1.INTENCLR.Set(nrf.RTC_INTENSET_COMPARE0)
+		nrf.RTC1.EVENTS_COMPARE[0].Set(0)
+		rtc_wakeup.Set(1)
+	})
+	intr.SetPriority(0xc0) // low priority
+	intr.Enable()
 }
 
 func putchar(c byte) {
@@ -96,10 +102,3 @@ func rtc_sleep(ticks uint32) {
 		arm.Asm("wfi")
 	}
 }
-
-//go:export RTC1_IRQHandler
-func handleRTC1() {
-	nrf.RTC1.INTENCLR.Set(nrf.RTC_INTENSET_COMPARE0)
-	nrf.RTC1.EVENTS_COMPARE[0].Set(0)
-	rtc_wakeup.Set(1)
-}