mips: adding MIPS64LE support to VEX

Necessary changes to VEX to support MIPS64LE on Linux.
Minor cleanup/style changes embedded in the patch as well.
Patch written by Dejan Jevtic and Petar Jovanovic.

More information about this issue:
https://bugs.kde.org/show_bug.cgi?id=313267


git-svn-id: svn://svn.valgrind.org/vex/trunk@2687 8f6e269a-dfd6-0310-a8e1-e2731360e62c