mips32: Fix a problem with gdb invoker.

This patch fixes the endian issue with gdb invoker on mips32
big endian. Now we are using pointer to long long and
we don't need to sign extend registers. On mips32 o32 abi
we need to make extra stack space when we are calling
function.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13507 a5019735-40e9-0310-863c-91ae7b9d1cf9