gobject: Assert that GObjects are at least as aligned as basic types

See the reasoning in the patch for why we believe GObjects *are*
(already) as aligned as the basic types.

We want to make this guarantee so that it’s guaranteed to be safe for
people to ignore -Wcast-align warnings for GObjects which contain basic
types. This typically happens with gdouble on 32-bit ARM platforms.

The checks are slightly complicated by the need to support GObjects with
custom constructors. We should expect that a custom construction
function will chain up to g_object_constructor (which calls
g_type_create_instance() as normal), but it’s possible that someone has
done something crazy and uses a custom allocator which doesn’t return
with the same alignment as GSlice. Hand them a warning in that case. If
that is true, the code which uses their custom-constructed GObject can
presumably already deal with the alignment it gets given.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1231

gobject/gobject.h

@@ -253,6 +253,10 @@ typedef void (*GWeakNotify)		(gpointer      data,
  * 
  * All the fields in the `GObject` structure are private to the implementation
  * and should never be accessed directly.
+ *
+ * Since GLib 2.72, all #GObjects are guaranteed to be aligned to at least the
+ * alignment of the largest basic GLib type (typically this is #guint64 or
+ * #gdouble).
  */
 struct  _GObject
 {