[cpp] Avoid write barriers for cpp.Function fields

[Tutez64]

Summary

Do not emit generational GC write barriers for cpp.Function instance fields.

With HXCPP_GC_GENERATIONAL enabled, assigning such a field generated an HX_OBJ_WB_SET call. This macro expects an object-like value with an mPtr member, but cpp::Function is a native function-pointer wrapper and does not have one, causing the generated C++ to fail to compile.

Fix

Exclude TCppFunction from GC write-barrier generation while keeping it classified as an object element for the other code paths that rely on that classification.

Tests

Added a C++ regression test which:

• Enables HXCPP_GC_GENERATIONAL
• Stores a native callback in a cpp.Function instance field
• Calls the stored callback and verifies its result

The targeted misc test passes successfully.

Closes #13001