mips64: fix 'unused variable' warning

On a couple of places, the code expected either _MIPSEB or _MIPSEL flag to
use some variables, but none of these flags is set when the code is compiled
for non-MIPS architectures.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2726 8f6e269a-dfd6-0310-a8e1-e2731360e62c