s390/cmpxchg: Implement arch_xchg() with arch_try_cmpxchg()

Get rid of the arch_xchg() inline assemblies by converting the inline
assemblies to C functions which make use of arch_try_cmpxchg().

With flag output operand support the generated code is at least as good as
the previous version. Without it is slightly worse, however getting rid of
all the inline assembly code is worth it.

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