s390: Support the SRNMT instruction.
Patch by Maran Pakkirisamy (maranp@linux.vnet.ibm.com).
Part of fixing BZ 307113.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2696 8f6e269a-dfd6-0310-a8e1-e2731360e62c