s390: make couple of variables and functions static

copy_oldmem_user() and ap_jumptable are private to the files they are
being used in. Therefore make them static.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>