ada: Fix latent alignment issue for dynamically-allocated controlled objects

ada: Fix latent alignment issue for dynamically-allocated controlled objects

Dynamically-allocated controlled objects are attached to a finalization
collection by means of a hidden header placed right before the object,
which means that the size effectively allocated must naturally account
for the size of this header.  But the allocation must also account for
the alignment of this header in order to have it properly aligned.

gcc/ada/

	* libgnat/s-finpri.ads (Header_Alignment): New function.
	(Header_Size): Adjust description.
	(Master_Node): Put Finalize_Address as first component.
	(Collection_Node): Likewise.
	* libgnat/s-finpri.adb (Header_Alignment): New function.
	(Header_Size): Return the object size in storage units.
	* libgnat/s-stposu.ads (Adjust_Controlled_Dereference): Replace
	collection node with header in description.
	* libgnat/s-stposu.adb (Adjust_Controlled_Dereference): Likewise.
	(Allocate_Any_Controlled): Likewise.  Pass the maximum of the
	specified alignment and that of the header to the allocator.
	(Deallocate_Any_Controlled): Likewise to the deallocator.