htl: Fix registration of atfork handlers in modules

We really need modules to use their own pthread_atfork so that
__dso_handle properly identifies them.

* sysdeps/htl/pt-atfork.c (__pthread_atfork): Hide function.
(pthread_atfork): Hide alias.
* sysdeps/htl/old_pt-atfork.c (pthread_atfork): Rename macro to
__pthread_atfork to fix building the compatibility alias.