If the exiting thread has a destructor registered that keeps
re-registering itself, the function loops forever. Implement
approximation for the number of passes over the desctructor list as it
was done before 9d26b82826d99.  Maintain the count of destructors
registered, and break out of the loop if we called more destructors then
the memoized count times CXA_DTORS_ITERATIONS.

__cxa_thread_call_dtors(3): bump number of iterations

This allows more breezing room for somewhat weird code that registers
thread-local destructors at destruction time.  At the same time, we
still limit the amount of iterations, making this aspect of the thread
termination time-bound.