ada: Streamline runtime support of finalization collections

Finalization collections are declared as (limited) controlled types so that
they can be naturally attached to a finalization master, but the same result
can be achieved by means of (limited) finalizable types, which need not be
tagged and thus avoid dragging the runtime support of tagged types.

gcc/ada/ChangeLog:

	* libgnat/s-finpri.ads: Remove clause for Ada.Finalization.
	(Finalization_Collection): Change to limited private type with the
	Finalizable aspect.
	(Initialize): Remove "overriding" keyword.
	(Finalize): Likewise.
	* libgnat/s-finpri.adb (Initialize): Likewise.
	(Finalize): Likewise.