Allow symbolizer to fall back to getStackTraceSafe

Summary: This makes sense on systems without `backtrace` and with LLVM libunwind implementations.

Reviewed By: yfeldblum

Differential Revision: D23625739

fbshipit-source-id: 6ec50c94c73bda227118ffc36d6856bcd8dd0d8b

folly/experimental/symbolizer/StackTrace.cpp

@@ -57,6 +57,8 @@ ssize_t getStackTrace(
 #elif FOLLY_HAVE_BACKTRACE
   int r = backtrace(reinterpret_cast<void**>(addresses), maxAddresses);
   return r < 0 ? -1 : r;
+#elif FOLLY_HAVE_LIBUNWIND
+  return getStackTraceSafe(addresses, maxAddresses);
 #else
   return -1;
 #endif