s390/futex: Generate futex atomic op functions

Cleanup the futex atomic op inline assembly and generate a function for
each futex atomic op. This makes the code hopefully a bit more readable.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>