ada: Remove the body of System.Storage_Elements

ada: Remove the body of System.Storage_Elements

All the subprograms declared in the unit have convention Intrinsic and
their current implementation makes some implicit assumptions that are
not valid universally, so it is replaced by a direct expansion.

This is mostly straightforward because Resolve_Intrinsic_Operator already
contains the required circuitry, but a few adjustements are necessary.

gcc/ada/

	* exp_ch4.adb (Expand_N_Op_Mod): Deal with the special mod
	operator of System.Storage_Elements.
	* exp_intr.adb (Expand_To_Integer): New procedure.
	(Expand_Intrinsic_Call): Call Expand_To_Integer appropriately.
	(Expand_To_Address): Deal with an argument with modular type.
	* sem_ch3.adb (Derive_Subprogram): Also set convention Intrinsic
	on a derived intrinsic subprogram.
	* sem_res.adb (Resolve_Arithmetic_Op): Deal with intrinsic
	operators not coming from source exactly as those coming from
	source and also generate a reference in both cases.
	(Resolve_Op_Expon): Likewise.
	(Resolve_Intrinsic_Operator): Call Implementation_Base_Type to get
	a nonprivate base type.
	* snames.ads-tmpl (Name_To_Integer): New intrinsic name.
	* libgnat/s-stoele.ads: Replace pragma Convention with pragma
	Import throughout and remove pragma Inline_Always and
	Pure_Function.
	* libgnat/s-stoele.adb: Replace entire contents with pragma
	No_Body.
	* libgnat/s-atacco.adb: Adjust comment about pragma No_Body.