Add missing padding from MemoryChunk struct

Add missing padding from MemoryChunk struct

Buildfarm animals skate, grison and mamba are Assert failing on the
pointer being given to repalloc not being MAXALIGNED.  c6e0fe1f2a made
changes in that area.

All of these animals are 32-bit with a MAXIMUM_ALIGNOF of 8 and a
SIZEOF_VOID_P of 4.  I suspect that the pointer is not properly aligned due
to the lack of padding in the MemoryChunk struct.

Here we add the same type of padding that was previously used in
AllocChunkData and GenerationChunk that c6e0fe1f2a neglected to add.

Discussion: https://postgr.es/m/CAA4eK1%2B1JyW5TiL%3DyV-3Uq1CrfnTyn0Xrk5uArt31Z%3D8rgPhXQ%40mail.gmail.com