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

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.