The test for futex support should be a test for Linux

Test: code inspection
Change-Id: Ic07f1eff2704064cc4416fc58df81c471ff54f4e
diff --git a/runtime/base/mutex.h b/runtime/base/mutex.h
index b0eb23d..1cf4ddd 100644
--- a/runtime/base/mutex.h
+++ b/runtime/base/mutex.h
@@ -31,10 +31,10 @@
 #include "base/globals.h"
 #include "base/macros.h"
 
-#if defined(__APPLE__)
-#define ART_USE_FUTEXES 0
-#else
+#if defined(__linux__)
 #define ART_USE_FUTEXES 1
+#else
+#define ART_USE_FUTEXES 0
 #endif
 
 // Currently Darwin doesn't support locks with timeouts.