score: Thread life cycle re-implementation

The thread deletion is now supported on SMP.

This change fixes the following PRs:

PR1814: SMP race condition between stack free and dispatch

PR2035: psxcancel reveals NULL pointer access in _Thread_queue_Extract()

The POSIX cleanup handler are now called in the right context (should be
called in the context of the terminating thread).

http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_09.html

Add a user extension the reflects a thread termination event.  This is
used to reclaim the Newlib reentrancy structure (may use file
operations), the POSIX cleanup handlers and the POSIX key destructors.

testsuites/psxtests/psxcleanup01/init.c

@@ -96,6 +96,15 @@ static void test_restart_with_cleanup(void)
   wait_for_restart_task();
 
   rtems_test_assert(restart_cleanup_arg == 1);
+
+  wait_for_restart_task();
+
+  sc = rtems_task_delete(id);
+  rtems_test_assert(sc == RTEMS_SUCCESSFUL);
+
+  wait_for_restart_task();
+
+  rtems_test_assert(restart_cleanup_arg == 2);
 }
 
 static void cleaner(void *arg)