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

src/include/utils/memutils_memorychunk.h

@@ -111,6 +111,14 @@ typedef struct MemoryChunk
 {
 #ifdef MEMORY_CONTEXT_CHECKING
 	Size		requested_size;
+#define MEMORYCHUNK_RAWSIZE		(SIZEOF_SIZE_T + 8)
+#else
+#define MEMORYCHUNK_RAWSIZE		8
+#endif							/* MEMORY_CONTEXT_CHECKING */
+
+	/* ensure proper alignment by adding padding if needed */
+#if (MEMORYCHUNK_RAWSIZE % MAXIMUM_ALIGNOF) != 0
+	char		padding[MAXIMUM_ALIGNOF - MEMORYCHUNK_RAWSIZE % MAXIMUM_ALIGNOF];
 #endif
 
 	/* bitfield for storing details about the chunk */