unw_backtrace() has two paths, tdap_trace() and slow_backtrace(), the latter of which strongly resembles what we do in LibunwindStepIteration. We intermittently see a SEGV in tdap_trace(), after which the stack is resolved by our internal LibunwindStepIteraion path.

Switch rawBacktrace() to slow_backtrace() emulation for correctness. This path is only used in SIGUSR2 handling, the heap profiler, and test code.