[3.13] pythongh-126312: Don't traverse frozen objects on the free-thr…

…eaded build (pythonGH-126338)

Also, _PyGC_Freeze() no longer freezes unreachable objects.

(cherry picked from commit d4c72fe)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>