Ok, I may have found the crucial piece of info:
The Nexus 9 runs on a 64-bit Dual Core processor, which makes it the first Android device to ship with a 64-bit ARM instruction set. Support for 64-bit processors was just added in Android 5.0, so if you have an NDK app, enable it by updating the APP_ABI value in your Application. mk file:
APP_ABI := armeabi armeabi-v7a arm64-v8a x86 x86_64 mips mips64
This would likely explain why you’re both getting the ERROR: Unknown NDK architecture ‘arm64-v8a’
Give that a try and let me know if it works.