The return value munging isn't necessary anymore. See for example
date: 2015/03/31 04:32:01; author: guenther; state: Exp; lines: +1 -10; commitid: 4lHzpIZZxMqyyel4;
Simplify fork/vfork logic: the kernel has handled returning zero in the child
for a long time, so there's no need to test the second return register here
in the asm stub.