ada: Fix address arithmetic issues in the expanded code

ada: Fix address arithmetic issues in the expanded code

This is most notably the addition of addresses in Expand_Interface_Thunk.
There is also a small change to Expand_Dispatching_Call, which was directly
accessing a class-wide interface object as a tag, thus giving rise later to
unchecked conversions between either the root or the equivalent record type
and access types.

gcc/ada/

	* exp_disp.adb (Expand_Dispatching_Call): In the abstract interface
	class-wide case, use 'Tag of the object as the controlling tag.
	(Expand_Interface_Thunk): Perform address arithmetic using operators
	of System.Storage_Elements.