Add implementation of System.out.println

With this, we can compile programs that actually have observable behavior and do what they're supposed to!

As for why I've opted to implement `println` in assembly: the code libfirm generates doesn't clear `%eax`, so the return value of `main` is implicitly whatever `%eax` contains. (Also, it's totally faster because of the missing function prologue and epilogue!!!1!1one)