s390/alternatives: Merge both alternative header files

s390/alternatives: Merge both alternative header files

The two alternative header files must stay in sync. This is easier to
achieve within one header file. Therefore merge both of them and have
only one file, like most other architectures.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Tested-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>