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_unix.go

@@ -24,6 +24,12 @@ type timespec struct {
 
 const CLOCK_MONOTONIC_RAW = 4
 
+func preinit() {
+}
+
+func postinit() {
+}
+
 func putchar(c byte) {
 	_Cfunc_putchar(int(c))
 }