Finalizers may not be called when flagged VM is terminated

[shirok]

GC queues finalizers, and it sets a flag in the VM of the sweeping thread. Once the thread resumes VM loop, it calls GC_invoke_finalizers() in a safe position, which processes the pending finalizers.

However, if the thread is terminated by another thread before that, the flag is never checked and the queued finalizers won't run.

Using a global flag involves high overhead (even lock-free check is costlier than localized memory reference).