Don't try to build the libcore native code on the Mac.

Change-Id: I79c5389af44ff2aea784d4f57c82d6aebf560132
diff --git a/NativeCode.mk b/NativeCode.mk
index 5ed3d1e..3a90afb 100644
--- a/NativeCode.mk
+++ b/NativeCode.mk
@@ -112,6 +112,8 @@
 # Build for the host.
 #
 
+ifeq ($(HOST_OS),linux)
+
 include $(CLEAR_VARS)
 LOCAL_CLANG := true
 LOCAL_SRC_FILES += $(core_src_files)
@@ -146,3 +148,5 @@
     LOCAL_CXX_STL := libc++
     include $(BUILD_HOST_SHARED_LIBRARY)
 endif # LIBCORE_SKIP_TESTS
+
+endif # HOST_OS == linux