[Ada] Do not compile predefined units with -gnatp in gnatmake

[Pierre-Marie de Rodat]

This aligns the behavior of gnatmake with the way the runtime is now
built.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * make.adb (GNAT_Flag): Change to "-gnatg".
        (Compile): Adjust comments accordingly.
diff --git a/gcc/ada/make.adb b/gcc/ada/make.adb
--- a/gcc/ada/make.adb
+++ b/gcc/ada/make.adb
@@ -464,7 +464,7 @@ package body Make is
    Ada_Flag_1        : constant String_Access := new String'("-x");
    Ada_Flag_2        : constant String_Access := new String'("ada");
    AdaSCIL_Flag      : constant String_Access := new String'("adascil");
-   GNAT_Flag         : constant String_Access := new String'("-gnatpg");
+   GNAT_Flag         : constant String_Access := new String'("-gnatg");
    Do_Not_Check_Flag : constant String_Access := new String'("-x");
 
    Object_Suffix : constant String := Get_Target_Object_Suffix.all;
@@ -1677,7 +1677,7 @@ package body Make is
          L            : File_Name_Type;
          Source_Index : Int;
          Args         : Argument_List) return Process_Id;
-      --  Compiles S using Args. If S is a GNAT predefined source "-gnatpg" is
+      --  Compiles S using Args. If S is a GNAT predefined source "-gnatg" is
       --  added to Args. Non blocking call. L corresponds to the expected
       --  library file name. Process_Id of the process spawned to execute the
       --  compilation.
@@ -2027,7 +2027,7 @@ package body Make is
             end loop;
          end;
 
-         --  Set -gnatpg for predefined files (for this purpose the renamings
+         --  Set -gnatg for predefined files (for this purpose the renamings
          --  such as Text_IO do not count as predefined). Note that we strip
          --  the directory name from the source file name because the call to
          --  Fname.Is_Predefined_File_Name cannot deal with directory prefixes.