Fix alignment requirements to include atomic u64.

Not having this led to bus errors on unaligned atomic u64 access
on architectures with 4B pointers.

lib/mempool.c

@@ -29,6 +29,7 @@ struct lp_chunk {
   struct lp_chunk *next;
   struct linpool *lp;
   uintptr_t data_align[0];
+  _Atomic u64 data_align_atomic[0];
   byte data[0];
 };
 

lib/resource.c

@@ -373,7 +373,10 @@ tmp_flush(void)
 struct mblock {
   resource r;
   unsigned size;
-  uintptr_t data_align[0];
+  union {
+    uintptr_t bigint;
+    _Atomic u64 atom;
+  } _align[0];
   byte data[0];
 };
 

lib/slab.c

@@ -171,6 +171,8 @@ struct sl_head {
 struct sl_alignment {			/* Magic structure for testing of alignment */
   byte data;
   int x[0];
+  _Atomic u64 y[0];
+  void *z[0];
 };
 
 #define TLIST_PREFIX sl_head