The existing overflow detection in VEX/priv/guest_ppc_toIR.c/set_XER_OV_64()

The existing overflow detection in VEX/priv/guest_ppc_toIR.c/set_XER_OV_64()
under the case PPCG_FLAG_OP_MULLW: does not apply to the mulldo as we need to
detect overflow when performing a Multiply Low Doubleword (not Multiply Low
Word). Hence, we added a new enumeration value PPCG_FLAG_OP_MULLD in
VEX/priv/guest_ppc_defs.h and a corresponding new case under which the
computation for detecting overflow for mulldo/mulldo. is added in
set_XER_OV_64(). The tests have been added to: none/tests/ppc32/jm-insns.c

Bugzilla 324594, submitted by Anmol P. Paralkar

Patch reviewed and tested by Carl Love

git-svn-id: svn://svn.valgrind.org/vex/trunk@2754