[Ada] New implementation for Normalize_Pathname

[Ada] New implementation for Normalize_Pathname

This implementation fixes an issue on Windows where a single drive letter
was not followed by a directory separator. On Windows the following
program:

   with Ada.Text_IO; use Ada.Text_IO;
   with GNAT.OS_Lib; use GNAT.OS_Lib;
   procedure Main is
   begin
      Put_Line (Normalize_Pathname ("c:\"));
      Put_Line (Normalize_Pathname ("c:\toto\.."));
   end Main;

Must output:

C:\
C:\

2018-01-11  Pascal Obry  <obry@adacore.com>

gcc/ada/

	* libgnat/s-os_lib.adb (Normalize_Pathname): New implementation.

From-SVN: r256501