mips32: Fix the align problem with mmap.

Valgrind is doing mmap always with MAP_FIXED. On mips32 we need to check arg4.
If the arg4 is MAP_SHARED we need to align the address to SHMLBA.
If the program tries to do mmap with VKI_FIXED Valgrind doesn't need to align
the address to SHMLBA.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13593 a5019735-40e9-0310-863c-91ae7b9d1cf9