C++11 thread_local destructors support

This feature is specifically for the C++ compiler to offload calling
thread_local object destructors on thread program exit, to glibc.
This is to overcome the possible complication of destructors of
thread_local objects getting called after the DSO in which they're
defined is unloaded by the dynamic linker.  The DSO is marked as
'unloadable' if it has a constructed thread_local object and marked as
'unloadable' again when all the constructed thread_local objects
defined in it are destroyed.

stdlib/exit.c

@@ -33,6 +33,9 @@ attribute_hidden
 __run_exit_handlers (int status, struct exit_function_list **listp,
 		     bool run_list_atexit)
 {
+  /* First, call the TLS destructors.  */
+  __call_tls_dtors ();
+
   /* We do it this way to handle recursive calls to exit () made by
      the functions registered with `atexit' and `on_exit'. We call
      everyone on the list and use the status value in the last