Add static instance of _on_exit_args for _REENT_SMALL platforms.

Add static instance of _on_exit_args for _REENT_SMALL platforms.

2015-12-21  Freddie Chopin  <freddie.chopin@gmail.com>

        * libc/stdlib/on_exit_args.{c,h}: New files.
        * libc/stdlib/Makefile.am: Add new source file.
        * libc/stdlib/Makefile.in: Regenerate.
        * libc/stdlib/__atexit.c (__register_exitproc): Initialize
        _on_exit_args_ptr field of _GLOBAL_ATEXIT on first run.
        * libc/stdlib/on_exit.c: Force linking of static instance of
        _on_exit_args.
        * libc/stdlib/cxa_atexit.c: Likewise.