mips64: add tests for Cavium BBIT0 and BBIT1

Follow up to VEX r2942 in which support for Cavium BBIT0/1 instructions
has been added.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14449 a5019735-40e9-0310-863c-91ae7b9d1cf9