Improved struct pmop alignment fix - avoid the slow path on 64 bit systems.

Commit c2a50ddb1bed6576 works round an alignment bug in the slab allocator
for 32 bit systems built with 64 bit IVs. However, the C pre-processor logic
meant that the test path was enabled on true 64 bit systems. It's not needed
there, so improve the logic so that it isn't compiled for platforms where
pointers are 64 bit.