ada: Remove dependency on System.Val_Bool in System.Img_Bool

ada: Remove dependency on System.Val_Bool in System.Img_Bool

In order to facilitate the certification of System.Img_Bool, remove
its dependency on unit System.Val_Bool. Modify the definition of
ghost function Is_Boolean_Image_Ghost to take the expected boolean
value and move it to System.Val_Spec.

gcc/ada/

	* libgnat/s-imgboo.adb: Remove with_clause now in spec file.
	* libgnat/s-imgboo.ads: Remove dependency on System.Val_Bool.
	(Image_Boolean): Replace call to Value_Boolean by passing value V
	to updated ghost function Is_Boolean_Image_Ghost.
	* libgnat/s-valboo.ads (Is_Boolean_Image_Ghost): Move to other
	unit.
	(Value_Boolean.): Update precondition.
	* libgnat/s-valspe.ads (Is_Boolean_Image_Ghost): Move here. Add
	new parameter for expected boolean value.