Only enable Metal support on 64-bit devices.

Prior to this commit, Cocos2D will still be built and run with Metal support if CC_ENABLE_METAL_RENDERING is 1 and we are building with the iOS 8 SDK, even if we are running on a device that doesn't support Metal such as the iPhone 5.

What really should happen is that if creating the Metal view fails then Cocos2D should fall back to OpenGL, instead of just crashing, but this will have to do for now.