s390: Fix s390_amode_for_guest_state. In general the offset relative 
to the guest state pointer may be more than the B12 addressing mode can
handle. Fall back and use a B20 addressing mode in those cases.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2817 8f6e269a-dfd6-0310-a8e1-e2731360e62c