ada: Diagnose too large size clause on floating-point type

The problem is that the size clause changes the floating-point format used
for the type, but it must not when this format is the widest format that is
supported in hardware on the target.  Instead a padding type must be built
and the associated warning given.

gcc/ada/

	* gcc-interface/decl.cc (gnat_to_gnu_entity): Cap the Esize of a
	floating-point type to the size of the widest format supported in
	hardware if it is explicity defined.