Allow overload on extern methods on all targets

[Simn]

This allows using overload methods which are marked extern or live on extern classes. The idea is that we want to allow this as long as we don't have to worry about generating the overloaded methods on targets that have no native support for it.

Some notes:

• Overloaded constructors are explicitly disallowed
• There are no other restrictions in place because the relevant restrictions should follow from the extern status
• @:overload is not supported.
• I'm adding one .disabled test which checks the presence of override. We currently don't run any checks like that on externs at all, so this may or may not be something to look into.

There are now only 4 checks for pf_overload left:

1. @:overload handling
2. Admit overloads for all methods, not just externs
3. Handle overloaded constructors
4. Don't call add_constructor because it doesn't support overloads

While the first two should remain in place, we could at some point look into supporting overloaded constructors as well. This can first be done for true externs, and maybe even extended to extern inline constructors afterwards.

I'm sure there are some missing checks here, but we can add those when we come across them.