[S390] Poison init section before freeing it.

The data patterns should allow us to easily tell if somebody accesses
initdata/code after it was freed. Same code as on various other
architectures.

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