use 64-bit semid_ds layout in the generic version of bits/sem.h

this layout is slightly less common than the old generic one, but only
because x86_64 and x32 wrongly (according to comments in the kernel
headers) copied the i386 padding. for future archs, and with 64-bit
time_t on 32-bit archs, the new layout here will become the most
common, and it makes sense to treat it as the generic.

arch/generic/bits/sem.h

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