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_atsamd51.go

@@ -6,6 +6,7 @@ import (
 	"device/arm"
 	"device/sam"
 	"machine"
+	"runtime/interrupt"
 	"runtime/volatile"
 )
 
@@ -202,8 +203,14 @@ func initRTC() {
 	for sam.RTC_MODE0.SYNCBUSY.HasBits(sam.RTC_MODE0_SYNCBUSY_ENABLE) {
 	}
 
-	arm.SetPriority(sam.IRQ_RTC, 0xc0)
-	arm.EnableIRQ(sam.IRQ_RTC)
+	irq := interrupt.New(sam.IRQ_RTC, func(interrupt.Interrupt) {
+		// disable IRQ for CMP0 compare
+		sam.RTC_MODE0.INTFLAG.SetBits(sam.RTC_MODE0_INTENSET_CMP0)
+
+		timerWakeup.Set(1)
+	})
+	irq.SetPriority(0xc0)
+	irq.Enable()
 }
 
 func waitForSync() {
@@ -271,14 +278,6 @@ func timerSleep(ticks uint32) {
 	}
 }
 
-//go:export RTC_IRQHandler
-func handleRTC() {
-	// disable IRQ for CMP0 compare
-	sam.RTC_MODE0.INTFLAG.SetBits(sam.RTC_MODE0_INTENSET_CMP0)
-
-	timerWakeup.Set(1)
-}
-
 func initUSBClock() {
 	// Turn on clock(s) for USB
 	//MCLK->APBBMASK.reg |= MCLK_APBBMASK_USB;