osx 64 bit does not want _ in variable names

BUG=none
TEST=build with m64
Review URL: http://webrtc-codereview.appspot.com/268011

git-svn-id: http://libyuv.googlecode.com/svn/trunk@81 16f28f9a-4ce2-e073-06de-1de4eb20be90

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 80
+Version: 81
 License: BSD
 License File: LICENSE
 

source/scale.cc

@@ -509,7 +509,7 @@ static void ScaleRowDown38_2_Int_NEON(const uint8* src_ptr, int src_stride,
     !defined(COVERAGE_ENABLED) && !TARGET_IPHONE_SIMULATOR
 #if defined(_MSC_VER)
 #define TALIGN16(t, var) __declspec(align(16)) t _ ## var
-#elif defined(OSX)
+#elif defined(OSX) && defined(__i386__)
 #define TALIGN16(t, var) t var __attribute__((aligned(16)))
 #else
 #define TALIGN16(t, var) t _ ## var __attribute__((aligned(16)))