Bug 328100 - XABORT not implemented.

XABORT can be called even when there is no current transaction.
In such a case XABORT acts as a NOP. Implement xabort as nop.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2800 8f6e269a-dfd6-0310-a8e1-e2731360e62c