s390: Add testcase for these DFP insns: CDGTRA, CXGTR, CGXTR, and CGDTR (VEX r2680).
Update vbit tester. Patch by Maran Pakkirisamy (maranp@linux.vnet.ibm.com).
Part of fixing BZ #307113.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13285 a5019735-40e9-0310-863c-91ae7b9d1cf9