s390/extable: search amode31 extable last

It is very unlikely that an exception happens within the amode31 text
section, therefore safe a couple of cycles for the common case, and
search the amode31 extable last.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>