Force the size and alignment of LWLock array entries to be either 16 or 32

Force the size and alignment of LWLock array entries to be either 16 or 32
bytes.  This shouldn't make any difference on x86 machines, where the size
happened to be 16 bytes anyway, but on 64-bit machines and machines with
slock_t int or wider, it will speed array indexing and hopefully reduce
SMP cache contention effects.  Per recent experimentation.