avr: add support for the digispark

Blinking the on-board LED works. Nothing else has been tested yet.

src/runtime/runtime_attiny.go

@@ -0,0 +1,16 @@
+// +build avr,attiny
+
+package runtime
+
+import (
+	"device/avr"
+)
+
+func sleepWDT(period uint8) {
+	// TODO: use the watchdog timer instead of a busy loop.
+	for i := 0x45; i != 0; i-- {
+		for i := 0xff; i != 0; i-- {
+			avr.Asm("nop")
+		}
+	}
+}