RFA: clean up logic in linux_child_wait

[Andrew Cagney]

> On Oct. 14, Mark Kettenis checked in a patch that was intended to make
> all Linux targets use lin-lwp instead of linux-thread.  The fact that
> S/390 still was afterwards was probably just an oversight; S/390 now
> contains the only reference to that file out of any port.


Probably me committing DJ Barrow's s390 changes.  If someone is trying 
to eliminate something, let me know and I'll add it to the ARI list.

	Andrew