nocode, noreturn

A more automatic approach to code suppression (aka. nocode_wanted)

The simple rules are:
- Clear 'nocode_wanted' at (im/explicit) label IF it was used
- Set 'nocode_wanted' after unconditional jumps

Also in order to test this then I did add the "function might
return no value" warning, and then to make that work again I
did add the __attribute__((noreturn)).

Also moved the look ahead label check into the type parser
to gain a little speed.

lib/va_list.c

@@ -60,6 +60,7 @@ void *__va_arg(__va_list_struct *ap,
 
     default: /* should never happen */
         abort();
+        return 0;
     }
 }
 #endif