mips32/mips64: Fix compiler warnings.

Fix some compiler warnings when compiling Valgrind for mips32/mips64.
Clean up exp files for mips32 BE and LE.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13496 a5019735-40e9-0310-863c-91ae7b9d1cf9