Disable sigaltstack() on macOS to make backtrace() work.

There are many reports that sigaltstack() on Apple platforms stops
backtrace() from tracing an original stack, e.g.

* d39ead4105
* https://reviews.llvm.org/D28265

If we disable an alternate stack for signal handler, we won't be able to
catch stack overflow errors, but backrace() will work again.

absl/debugging/failure_signal_handler.cc

@@ -50,7 +50,9 @@
 #ifndef _WIN32
 #define ABSL_HAVE_SIGACTION
 // Apple WatchOS and TVOS don't allow sigaltstack
-#if !(defined(TARGET_OS_WATCH) && TARGET_OS_WATCH) && \
+// Apple macOS has sigaltstack, but using it makes backtrace() unusable.
+#if !(defined(TARGET_OS_OSX) && TARGET_OS_OSX) && \
+    !(defined(TARGET_OS_WATCH) && TARGET_OS_WATCH) && \
     !(defined(TARGET_OS_TV) && TARGET_OS_TV) && !defined(__QNX__)
 #define ABSL_HAVE_SIGALTSTACK
 #endif