s390/string: use generic strrchr

Use generic strrchr instead of an optimized architecture specific
variant. Performance of strrchr is not relevant for real life
workloads, since the only user which may call this more frequently
would be kbasename().

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/lkml/CAHk-=whoe211F8ND-9hZvfnib0UA4gga8DZJ+YaBZNbE4fubdg@mail.gmail.com/


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>