ada: Finalization_Size raises Constraint_Error

When the attribute Finalization_Size is applied to an interface type
object, the compiler-generated code fails at runtime, raising a
Constraint_Error exception.

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference) <Finalization_Size>:
	If the prefix is an interface type, generate code to obtain its
	address and displace it to reference the base of the object.