[Ada] Reduce runtime dependencies on stage1

gcc/ada/

	* osint.adb (File_Names_Equal): Declare To_Lower locally.
	* osint.ads (Null_FD): New.
	* fmap.adb, sinput-l.adb, targparm.adb: Adapt to changes above.
	* switch-b.adb (Scan_Debug_Switches): Use Is_Regular_File to
	simplify the bootstrap dependencies.

gcc/ada/fmap.adb

@@ -301,7 +301,7 @@ package body Fmap is
       Read_Source_File (Name_Enter (File_Name), 1, Hi, Src, FD, Config);
 
       if Null_Source_Buffer_Ptr (Src) then
-         if FD = Null_FD then
+         if FD = Osint.Null_FD then
             Write_Str ("warning: could not locate mapping file """);
          else
             Write_Str ("warning: no read access for mapping file """);

gcc/ada/osint.adb

@@ -1059,6 +1059,21 @@ package body Osint is
    ----------------------
 
    function File_Names_Equal (File1, File2 : String) return Boolean is
+
+      function To_Lower (A : String) return String;
+      --  For bootstrap reasons, we cannot use To_Lower from System.Case_Util
+
+      --------------
+      -- To_Lower --
+      --------------
+
+      function To_Lower (A : String) return String is
+         Result : String := A;
+      begin
+         To_Lower (Result);
+         return Result;
+      end To_Lower;
+
    begin
       if File_Names_Case_Sensitive then
          return File1 = File2;

gcc/ada/osint.ads

@@ -52,6 +52,10 @@ package Osint is
    Project_Include_Path_File : constant String := "ADA_PRJ_INCLUDE_FILE";
    Project_Objects_Path_File : constant String := "ADA_PRJ_OBJECTS_FILE";
 
+   Null_FD : constant File_Descriptor := -2;
+   --  Uninitialized file descriptor. Copied from System.OS_Lib for bootstrap
+   --  reasons.
+
    Output_FD : File_Descriptor;
    --  File descriptor for current library info, list, tree, C, H, or binder
    --  output. Only one of these is open at a time, so we need only one FD.

gcc/ada/sinput-l.adb

@@ -420,7 +420,7 @@ package body Sinput.L is
       if Null_Source_Buffer_Ptr (Src) then
          Source_File.Decrement_Last;
 
-         if FD = Null_FD then
+         if FD = Osint.Null_FD then
             return No_Source_File;
          else
             return No_Access_To_Source_File;

gcc/ada/switch-b.adb

@@ -312,7 +312,7 @@ package body Switch.B is
 
             Ptr := Max + 1;
 
-            if not Is_Read_Accessible_File (Force_Elab_Order_File.all) then
+            if not Is_Regular_File (Force_Elab_Order_File.all) then
                Osint.Fail (Force_Elab_Order_File.all & ": file not found");
             end if;
 

gcc/ada/targparm.adb

@@ -165,7 +165,7 @@ package body Targparm is
       if Null_Source_Buffer_Ptr (Text) then
          Write_Line ("fatal error, run-time library not installed correctly");
 
-         if FD = Null_FD then
+         if FD = Osint.Null_FD then
             Write_Line ("cannot locate file system.ads");
          else
             Write_Line ("no read access for file system.ads");