[S390] s390: use PAGE_SIZE in vmlinux.lds

Replace the hardcoded 4096 value with the PAGE_SIZE macro.
Converted a few decimal numbers to readable hex numbers.

Use of PAGE_SIZE required a small change to page.h
to allow PAGE_SIZE to be used from assembler/linker scripts.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>