avr: convert initialization from asm to Go

This increases code size by 1 instruction (2 bytes) because LLVM isn't
yet smart enough to recognize that it doesn't need to clear a register
to use 0: it can just use r1 which is always 0 according to the
convention. It makes initialization a lot easier to read, however.

src/runtime/runtime_nrf.go

@@ -16,6 +16,12 @@ func _start() {
 	main()
 }
 
+func preinit() {
+}
+
+func postinit() {
+}
+
 func init() {
 	initUART()
 	initLFCLK()