hurd: Avoid PLT ref for __pthread_get_cleanup_stack

	* htl/pt-cleanup.c (___pthread_get_cleanup_stack): Rename to
	__pthread_get_cleanup_stack.
	(__pthread_get_cleanup_stack): Remove alias, add hidden def.
	* htl/pt-exit.c (__pthread_exit): Use __pthread_get_cleanup_stack
	instead of ___pthread_get_cleanup_stack.
	* sysdeps/htl/pthread-functions.h [libpthread]
	(__pthread_get_cleanup_stack): Add hidden proto.
	* sysdeps/htl/pthreadP.h (___pthread_get_cleanup_stack): Remove
	prototype.