mips: fix for some warnings in mips files when compiling on amd64/x86

Fixing some constant size related warnings in mips files on gcc20(amd64).
Thanks Philippe for reporting it.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2689 8f6e269a-dfd6-0310-a8e1-e2731360e62c