use backtrace(2) on all Apple platforms

Summary: I don't recall what previous intermediate state justified guarding use of backtrace with __APPLE__ and FOLLY_USE_SYMBOLIZER, but it no longer applies.

Reviewed By: yfeldblum, luciang

Differential Revision: D23881192

fbshipit-source-id: b3ca829f9c738f50f7ab7e038348ec3df65ae4b9

folly/experimental/symbolizer/StackTrace.cpp

@@ -126,7 +126,7 @@ ssize_t getStackTraceInPlace(
 ssize_t getStackTraceSafe(
     FOLLY_MAYBE_UNUSED uintptr_t* addresses,
     FOLLY_MAYBE_UNUSED size_t maxAddresses) {
-#if defined(__APPLE__) && FOLLY_USE_SYMBOLIZER
+#if defined(__APPLE__)
   // While Apple platforms support libunwind, the unw_init_local,
   // unw_step step loop does not cross the boundary from async signal
   // handlers to the aborting code, while `backtrace` from execinfo.h