fix: generate valid function pointers for embedded type's unexported methods (#1597)

Previously, when a type embedded another type from a different package,
the unexported methods inherited from the embedded type would have nil
function pointers (Ifn_) in the method table. This caused runtime panics
when interfaces with unexported methods (like types.Object) were used.

The fix removes the skipfn logic that was setting Ifn_ to nil for
methods from other packages. Now all methods, including unexported ones
from embedded types, get valid function pointers generated.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: luoliwoshang