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.

newlib/ChangeLog

@@ -1,3 +1,8 @@
+2015-12-21  Freddie Chopin  <freddie.chopin@gmail.com>
+
+	* libc/stdlib/__atexit.c (__register_exitproc): Fix for
+	_ATEXIT_DYNAMIC_ALLOC.
+
 2015-12-21  Freddie Chopin  <freddie.chopin@gmail.com>
 
 	* libc/stdlib/on_exit_args.{c,h}: New files.

newlib/libc/stdlib/__atexit.c

@@ -131,6 +131,12 @@ _DEFUN (__register_exitproc,
       args = p->_on_exit_args_ptr;
       if (args == NULL)
 	{
+#ifndef _ATEXIT_DYNAMIC_ALLOC
+#ifndef __SINGLE_THREAD__
+	  __lock_release_recursive(__atexit_lock);
+#endif
+	  return -1;
+#else
 	  if (malloc)
 	    args = malloc (sizeof * p->_on_exit_args_ptr);
 
@@ -144,6 +150,7 @@ _DEFUN (__register_exitproc,
 	  args->_fntypes = 0;
 	  args->_is_cxa = 0;
 	  p->_on_exit_args_ptr = args;
+#endif
 	}
 #else
       args = &p->_on_exit_args;