cygwin: clear screen using wide char function

FillConsoleOutputCharacterA doesn't work correctly in codepage 65001
(UTF-8).  Looks like the character conversion function from ascii char
to unicode char works incorrectly then.  Use FillConsoleOutputCharacterW
instead.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler_console.cc

@@ -1319,7 +1319,7 @@ dev_console::clear_screen (HANDLE h, int x1, int y1, int x2, int y2)
       tlc.X = x2;
       tlc.Y = y2;
     }
-  FillConsoleOutputCharacterA (h, ' ', num, tlc, &done);
+  FillConsoleOutputCharacterW (h, L' ', num, tlc, &done);
   FillConsoleOutputAttribute (h, current_win32_attr, num, tlc, &done);
 }