s390x: set alignment for long to 8

The alignment for longs on s390x is 8. That's the only place where it differs
from the default alignments found in configure already. The example alignment
program from Laurent printed the following on a real s390x:

  alignof(short) 2
  alignof(int) 4
  alignof(long) 8
  alignof(long long) 8

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>