s390/uaccess: Make cmpxchg_user_key() library code

Move cmpxchg_user_key() handling to uaccess library code. The generated
code is large in any case so that there is hardly any benefit if it is
inlined.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>