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.go

@@ -22,9 +22,15 @@ var hasScheduler bool
 // Entry point for Go. Initialize all packages and call main.main().
 //go:export main
 func main() int {
+	// Initialize memory etc.
+	preinit()
+
 	// Run initializers of all packages.
 	initAll()
 
+	// Enable interrupts etc.
+	postinit()
+
 	// This branch must be optimized away. Only one of the targets must remain,
 	// or there will be link errors.
 	if hasScheduler {