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/machine/machine_nrf.go

@@ -3,9 +3,9 @@
 package machine
 
 import (
-	"device/arm"
 	"device/nrf"
 	"errors"
+	"runtime/interrupt"
 )
 
 var (
@@ -88,8 +88,9 @@ func (uart UART) Configure(config UARTConfig) {
 	nrf.UART0.INTENSET.Set(nrf.UART_INTENSET_RXDRDY_Msk)
 
 	// Enable RX IRQ.
-	arm.SetPriority(nrf.IRQ_UART0, 0xc0) // low priority
-	arm.EnableIRQ(nrf.IRQ_UART0)
+	intr := interrupt.New(nrf.IRQ_UART0, UART0.handleInterrupt)
+	intr.SetPriority(0xc0) // low priority
+	intr.Enable()
 }
 
 // SetBaudRate sets the communication speed for the UART.
@@ -116,7 +117,7 @@ func (uart UART) WriteByte(c byte) error {
 	return nil
 }
 
-func (uart UART) handleInterrupt() {
+func (uart *UART) handleInterrupt(interrupt.Interrupt) {
 	if nrf.UART0.EVENTS_RXDRDY.Get() != 0 {
 		uart.Receive(byte(nrf.UART0.RXD.Get()))
 		nrf.UART0.EVENTS_RXDRDY.Set(0x0)