Make sure to always inline __xchg() and __cmpxchg() otherwise the
compiler might decide to generate out-of-line versions which will
fail at link time:

   s390-linux-ld: lib/atomic64_test.o: in function `__xchg':
>> atomic64_test.c:(.text.unlikely+0xa4): undefined reference to `__xchg_called_with_bad_pointer'
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/linux-mm/202104170449.SIIFKVjT-lkp@intel.com/
Fixes: d2b1f6d2 ("s390/cmpxchg: get rid of gcc atomic builtins")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>