ada: Fix address arithmetic issues in the runtime

ada: Fix address arithmetic issues in the runtime

This is most notably the addition of addresses in Interfaces.C.Pointers and
System.Bitfield_Utils.  There is also a change to System.Stream_Attributes,
which was representing a thin pointer as a record, which is not problematic
per se, but is in the end, because the expanded code performs an unchecked
conversion from it to the access type instead of accessing the component.

gcc/ada/

	* libgnat/i-cpoint.adb: Add clauses for System.Storage_Elements.
	(Addr): Delete.
	(Offset): New subtype of Storage_Offset.
	(To_Offset): New instance of Unchecked_Conversion.
	(To_Pointer): Adjust.
	(To_Addr): Likewise.
	(To_Ptrdiff): Likewise.
	("+"): Call To_Offset on the offset.
	("-"): Likewise.
	* libgnat/s-bituti.adb: Add clauses for System.Storage_Elements.
	(Val_Bytes): Change type to Storage_Count.
	(Get_Val_2): Add qualification to second operand of mod operator.
	(Set_Val_2): Likewise.
	(Copy_Bitfield): Likewise.  Change type of Src_Adjust & Dest_Adjust.
	* libgnat/s-stratt.ads (Thin_Pointer): Change to subtype of Address.
	* libgnat/s-statxd.adb (I_AD): Adjust.
	(I_AS): Likewise.
	(W_AS): Likewise.