[S390] bug: use relative pointers in bug table entries

Reduces the size of the bug table entries by 50% on 64bit kernels.
Saves around 30kb on a defconfig kernel.
s390 version of b93a531e "allow bug table entries to use relative
pointers (and use it on x86-64)".

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