Mac doesn't define MAP_ANONYMOUS

Fix mac build.

Change-Id: I607e8fff227259fbf27e468193a705c8c3fc1b59
diff --git a/runtime/mem_map.cc b/runtime/mem_map.cc
index 6c1c867..8d987df 100644
--- a/runtime/mem_map.cc
+++ b/runtime/mem_map.cc
@@ -39,6 +39,10 @@
 #endif
 #endif
 
+#ifndef MAP_ANONYMOUS
+#define MAP_ANONYMOUS MAP_ANON
+#endif
+
 namespace art {
 
 static std::ostream& operator<<(