[Ada] Remove use of use-clauses in loaded runtime units

[Ada] Remove use of use-clauses in loaded runtime units

The spec of runtime units that may be loaded by the compiler should not
contain use-clauses, for visibility to be correctly handled.  Remove
use-clauses that were introduced for the ghost big integers unit as part
of the proof of runtime units.

gcc/ada/

	* libgnat/s-aridou.ads: Remove use-clause, add renames and
	subtypes.
	* libgnat/s-exponn.ads: Same.
	* libgnat/s-expont.ads: Same.
	* libgnat/s-widthu.ads: Same.