unw_backtrace is not async-signal-safe

unw_backtrace is not async-signal-safe

Summary:
Even though it should be according to the docs; tdep_trace allocates memory on
first use from each thread.

Wrote a slow loop that we can use from the signal handler. The exception tracer
still uses the fast version.

Test Plan: fbconfig -r folly/experimental/symbolizer folly/experimental/exception_tracer && fbmake runtests_opt

Reviewed By: philipp@fb.com

FB internal diff: D1101095