hurd: Fix missing __pthread_get_cleanup_stack symbol

Rework 57e16515 ("hurd: Avoid PLT ref for __pthread_get_cleanup_stack")
to keep the __pthread_get_cleanup_stack symbol.

        * htl/pt-cleanup.c (__pthread_get_cleanup_stack): Rename to
        ___pthread_get_cleanup_stack.
        (__pthread_get_cleanup_stack): Remove hidden def, add alias.
        * 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): Remove hidden proto.
        * sysdeps/htl/pthreadP.h (___pthread_get_cleanup_stack): Add
        prototype.
	* sysdeps/htl/pthreadP.h (___pthread_get_cleanup_stack): Add hidden
	attribute.
	* htl/pt-join.c (__pthread_get_cleanup_stack): Define to
	___pthread_get_cleanup_stack.