[Android] Enable x86 64-bit support for Crosswalk

BUG=XWALK-2839
@wang16  Yes, I notice that. But I only found the definition for x86 and arm, no x86_64, from this page, published in 2012, https://software.intel.com/en-us/blogs/2012/11/12/how-to-publish-your-apps-on-google-play-for-x86-based-android-devices-using
Anybody know the right number?