ada: Propagate aspect Ghost when instantiating null formal procedures

ada: Propagate aspect Ghost when instantiating null formal procedures

When instantiating generic package that includes a formal subprogram
declaration with Ghost aspect and a subprogram_default of null, e.g.:

   generic
     with procedure Proc is null with Ghost;
   package P is ...

the Ghost aspect should be propagated to the internally generated null
subprogram, so this null subprogram can be used in contexts that require
ghost entities.

gcc/ada/

	* sem_ch12.adb (Instantiate_Formal_Subprogram): Copy aspect Ghost
	from formal subprogram declaration to the internally generated
	procedure.