Fix backtrace

Sergey Kosukhinrequested to merge
fix-backtrace-consistency into master
  1. Provide backtrace on macOS via execinfo.h.
  2. Fix unwind-based backtrace for non-PIE.
  3. Support unwind-based backtrace for PIE.
  4. Drop redundant dependency on POSIX threads.
  5. Clean up the backtrace functionality.

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