mips64: implement Cavium BBIT0 and BBIT1 instructions

Implement Cavium specific instructions:
- BBIT0 (Branch on bit clear)
- BBIT1 (Branch on bit set)

This should fix the hang issue reported in:
https://bugs.kde.org/show_bug.cgi?id=336139


git-svn-id: svn://svn.valgrind.org/vex@2942 8f6e269a-dfd6-0310-a8e1-e2731360e62c