avr: fix time.Sleep() in init code

In the early days of TinyGo, the idea of `postinit` was to enable
interrupts only after initializers have run. Which kind of makes
sense... except that `time.Sleep` is allowed in init code and
`time.Sleep` requires interrupts to be enabled. Therefore, interrupts
must be enabled while initializers are being run.

This commit simply moves the enabling of interrupts to a point right
before running package initializers. It also removes `runtime.postinit`,
which is not necessary anymore (and was only used on AVR).

src/runtime/runtime_avr.go

@@ -42,6 +42,7 @@ var _ebss [0]byte
 //export main
 func main() {
 	preinit()
+	initHardware()
 	run()
 	exit(0)
 }
@@ -55,15 +56,13 @@ func preinit() {
 	}
 }
 
-func postinit() {
-	// Enable interrupts after initialization.
-	avr.Asm("sei")
-}
-
-func init() {
+func initHardware() {
 	initUART()
 	machine.InitMonotonicTimer()
 	nextTimerRecalibrate = ticks() + timerRecalibrateInterval
+
+	// Enable interrupts after initialization.
+	avr.Asm("sei")
 }
 
 func ticksToNanoseconds(ticks timeUnit) int64 {