fix semid_ds structure on mips

This used to be broken when all archs had the same semid_ds definition:
there is no padding around the time_t members on mips.
fix statfs struct on mips

The mips statfs struct layout is different than on other archs, so the
statfs, fstatfs, statvfs and fstatvfs APIs were broken on mips.
Now the ordering is fixed, the types are kept consistent with other archs.