[S390] xchg/cmpxchg: move to own header file

Move xchg() and cmpxchg() functions to own header file like some
other architectures have done.
With this we make sure that system.h now really looks like a place
where everything is gathered that doesn't fit anywhere else.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>