[Ada] Put_Image: Enable for access-to-subprogram types

[Ada] Put_Image: Enable for access-to-subprogram types

2020-06-15  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_put_image.adb, libgnat/s-putima.adb, libgnat/s-putima.ads,
	rtsfind.ads: Enable Put_Image if Is_Access_Subprogram_Type (Typ).
	Remove comment saying it's disabled in that case.  Rename
	Put_Image_Access_Prot to be Put_Image_Access_Prot_Subp to
	clarify that we're talking about access-to-subprogram, not
	access-to-protected-object.