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.

move struct semid_ds to from shared sys/sem.h to bits

the definition was found to be incorrect at least for powerpc, and
fixing this cleanly requires making the definition arch-specific. this
will allow cleaning up the definition for other archs to make it more
specific, and reversing some of the ugliness (time_t hacks) introduced
with the x32 port.

this first commit simply copies the existing definition to each arch
without any changes. this is intentional, to make it easier to review
changes made on a per-arch basis.