s390: cleanup arch/s390/kernel Makefile

Group all compiler flag modification lines together and sort them
alphabetically. This should hopefully prevent future bugs due to
missing flag modifications.
Also fix indentation at some places.

Reviewed-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>