s390/linker skript: discard exit.data at runtime

Discard exit.data section at run time, not link time, since exit.text
references exit.data and causes this build error:

`.exit.data' referenced in section `.exit.text' of drivers/built-in.o:
 defined in discarded section `.exit.data' of drivers/built-in.o

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