See if running single-threaded fixes dex2oat Mac crashes.

Change-Id: Ic5fce7d5eec47b4051e7c93eb1a739aede3b7634
diff --git a/src/dex2oat.cc b/src/dex2oat.cc
index 28960b9..c96620e 100644
--- a/src/dex2oat.cc
+++ b/src/dex2oat.cc
@@ -493,7 +493,7 @@
   uintptr_t image_base = 0;
   UniquePtr<std::string> host_prefix;
   std::vector<const char*> runtime_args;
-#if defined(ART_USE_QUICK_COMPILER)
+#if defined(ART_USE_QUICK_COMPILER) || defined(__APPLE__)
   int thread_count = 1;
 #else
   int thread_count = sysconf(_SC_NPROCESSORS_CONF);