s390: First round of changes to support the PFPO insn.
Support these IROps:
Iop_F64toD64, Iop_D64toF64
Iop_F64toD128, Iop_D128toF64,
Iop_F128toD128, Iop_D128toF128,

Patch by Maran Pakkirisamy (maranp@linux.vnet.ibm.com).
Part of fixing BZ #307113


git-svn-id: svn://svn.valgrind.org/vex/trunk@2719 8f6e269a-dfd6-0310-a8e1-e2731360e62c