ada: Finish up support for relaxed finalization

ada: Finish up support for relaxed finalization

This adds a variant of the System.Finalization_Primitives unit that supports
only controlled types with relaxed finalization, and adds the description of
its implementation to Exp_Ch7.

gcc/ada/

	* exp_ch7.adb (Relaxed Finalization): New paragraph in head
	comment.
	* sem_ch13.adb (Validate_Finalizable_Aspect): Give an error
	message if strict finalization is required but not supported by
	the runtime.