4x function API's call_function cannot call a 3x function

The 4x API exposes a method called "call_function" that a 4x function can use to call other functions.

This is however currently restricted to only work for 4x functions as the function is loaded using the loaders subsystem, and this subsystem is not used to load 3x functions.

We can either change the use of the loaders for 3x functions, or add handling in the call_function to perform lookup of a 3x function the "old way" if it is not found in the new loaders subsystem.