s390: Support the following DFP insns:
- extract basied exponent
- insert biased exponent
- quantize
- reround to significance
Patch by Maran Pakkirisamy (maranp@linux.vnet.ibm.com).
Part of fixing BZ #307113.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2684 8f6e269a-dfd6-0310-a8e1-e2731360e62c