Zend: follow-up for GH-22145. Fix zend_fcc_closure_equals_ex() implementation

[Girgias]

This is effectively a follow-up to #22145 to properly fix #22118 with internal functions.

It turns out that to prevent memory leaks for fake closures of internal functions, the zif_handler is changed to zend_closure_internal_handler and the true zif_handler is stored in the orig_internal_handler field of the zend_closure struct.

So return func1->internal_function.handler == func2->internal_function.handler; could never return true for internal functions.

We need to keep zend_fcc_closure_equals_ex() as the ABI is frozen, but it probably makes sense to clean this all up in master.

I'll definitely write some docs about zend_closure after I finished the FCI/FCC ones as this is incredibly confusing.