lib/cloner.c: Use clone() on s390 and s390x too

There is no reason to use undocumented __clone() on s390 and s390x
as I haven't been able to find a distribution where clone() is not
defined on these. I suppose that this is a leftover from early clone
implementation days.

Moreover it fixes build failure on recent enough s390 and s390x where
clone accepts seven aruments. (i.e. SLES11 and newer)

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>