You need device support from previous version of Xcode (7.3.1).
I would recommend to use this tool: https://github.com/KrauseFx/xcode-install
When you install Xcode 7, you should link support files to Xcode 8:
sudo ln -s /Applications/Xcode-7.3.1.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/7.* \
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/