[Ada] Code and style cleanups for CUDA

[Ada] Code and style cleanups for CUDA

gcc/ada/

	* exp_prag.adb (Expand_Pragma_CUDA_Execute): Refill comments;
	remove periods after single-line comments; use procedural
	variant of Next_Entity.
	* gnat_cuda.adb: Refill comments; remove periods after
	single-line comments; replace calls to UI_From_Int with
	constants; change iteration bounds so they match the comments.
	* sem_prag.adb (Analyze_Pragma): Add checks for malformed pragma
	CUDA_Kernel aggregate; simplify processing of pragma CUDA_Global
	with Check_Arg_Count; sync comment with code for CUDA_Global.