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.

arch/mips/bits/sem.h

 struct semid_ds {
 	struct ipc_perm sem_perm;
 	time_t sem_otime;
-	time_t __unused1;
 	time_t sem_ctime;
-	time_t __unused2;
 #if __BYTE_ORDER == __LITTLE_ENDIAN
 	unsigned short sem_nsems;
 	char __sem_nsems_pad[sizeof(time_t)-sizeof(short)];