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.

tools/gen-device-avr/gen-device-avr.go

@@ -260,6 +260,7 @@ func writeGo(outdir string, device *Device) error {
 package {{.pkgName}}
 
 import (
+	"runtime/interrupt"
 	"runtime/volatile"
 	"unsafe"
 )
@@ -277,6 +278,12 @@ const ({{range .interrupts}}
 	IRQ_max = {{.interruptMax}} // Highest interrupt number on this device.
 )
 
+// Map interrupt numbers to function names.
+// These aren't real calls, they're removed by the compiler.
+var ({{range .interrupts}}
+	_ = interrupt.Register(IRQ_{{.Name}}, "__vector_{{.Name}}"){{end}}
+)
+
 // Peripherals.
 var ({{range .peripherals}}
 	// {{.Caption}}