[S390] standardize Kbuild rules

Introducing this Kbuild file allow us to:

make arch/s390/

And thus building all the core part of s390.
Same as on other architectures.

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