[Ada] Improve CUDA host-side and device-side binder support

[Ada] Improve CUDA host-side and device-side binder support

On the host-side, the binder now generates CUDA_Execute pragmas to
invoke initialization and finalization on the device-side. The
front end's expansion of these pragmas includes references to
entities declared in package CUDA.Internal. Generate a with-clause so
that unit will be available.

gcc/ada/

	* bindgen.adb: When the binder is invoked for the host, generate a
	"with CUDA.Internal;" with clause.