Fix Unix exception handling in finalizers

Fix Unix exception handling in finalizers

When unhandled exception happens in a finalizer thread and there are
no managed frames till the bottom of the stack,
the Thread::VirtualUnwindToFirstManagedCallFrame then fails fast.
The fix is to make the Thread::VirtualUnwindToFirstManagedCallFrame to
return even in case no managed frame is called, which is indicated by
its returning 0 as the resulting IP address. The DispatchManagedException
then checks that and rethrows the exception instead of trying to call
UnwindManagedExceptionPass1.
I have also added INSTALL_UNHANDLED_MANAGED_EXCEPTION_TRAP to the
FinalizerThread::FinalizerThreadStart so that the unhandled exception
doesn't escape the stack.
And I also needed to modify the UNINSTALL_UNHANDLED_MANAGED_EXCEPTION_TRAP
to detect case when the unhandled exception filter was already executed so
that it doesn't double-report the unhandled exception.