Make slab allocator work on platforms with MAXIMUM_ALIGNOF < sizeof(int).

Notably, m68k only needs 2-byte alignment. Per report from Christoph Berg.

Discussion: https://www.postgresql.org/message-id/20170517193957.fwntkgi6epuso5l2@msg.df7cb.de

src/backend/utils/mmgr/slab.c

@@ -194,9 +194,9 @@ SlabContextCreate(MemoryContext parent,
 					 MAXALIGN(sizeof(SlabChunk)),
 					 "padding calculation in SlabChunk is wrong");
 
-	/* otherwise the linked list inside freed chunk isn't guaranteed to fit */
+	/* Make sure the linked list node fits inside a freed chunk */
-	StaticAssertStmt(MAXIMUM_ALIGNOF >= sizeof(int),
+	if (chunkSize < sizeof(int))
-					 "MAXALIGN too small to fit int32");
+		chunkSize = sizeof(int);
 
 	/* chunk, including SLAB header (both addresses nicely aligned) */
 	fullChunkSize = MAXALIGN(sizeof(SlabChunk) + MAXALIGN(chunkSize));