Build foreign objects for TH with interpreter's way when loading from iface

Fixes #25211

When linking bytecode for TH from interface core bindings with
`-fprefer-byte-code`, foreign sources are loaded from the interface as
well and compiled to object code in an ad-hoc manner.

The results are then loaded by the interpreter, whose way may differ
from the current build's target way.

This patch ensures that foreign objects are compiled with the
interpreter's way.