When observer is enabled, we normally add an extra temporary to all
functions, to store the previously observed frame. However, this is done in
zend_observer_post_startup() so it doesn't happen to dl'ed() functions.

One possible fix would be to move that from zend_observer_post_startup()
to zend_register_functions(), but this would be too early: Observer may
not be enabled when zend_register_functions() is called, and may still be
enabled later.

However, when zend_register_functions() is called at run-time (during dl()),
we know definitively whether observer is enabled.

Here I update zend_register_functions() to add a temporary to dl'ed()
functions when observer is enabled.

Fixes: GH-17211
Closes: GH-17220