Fix atexit logic to honor _ATEXIT_DYNAMIC_ALLOC setting.

Fix atexit logic to honor _ATEXIT_DYNAMIC_ALLOC setting.

If small reent is enabled (_REENT_SMALL is defined) then malloc() was
used in __register_exitproc() even if user requested it to be disabled
(_ATEXIT_DYNAMIC_ALLOC is defined). With this fix, function fails when
_ATEXIT_DYNAMIC_ALLOC is defined and whole static storage is already
used.

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

        * libc/stdlib/__atexit.c (__register_exitproc): Fix for
        _ATEXIT_DYNAMIC_ALLOC.