[sgen] Incomplete bridge weak reference fix.

[sgen] Incomplete bridge weak reference fix.

This is an incomplete fix for #6715.  After bridge processing we
iterate through all weak references again, this time nulling those
that point to bridged objects that were diagnosed as dead by the
bridge.

As of now it's inefficient because it uses linear search for each weak
reference lookup, as well as incomplete because the code runs while
the world is running, so the weak links might be accessed before they
are consistent.