Only generate ptx code for the latest supported arch
Only generate ptx code for the latest supported arch

Follows recommendation of njuffa from NVIDIA:
http://forums.nvidia.com/index.php?showtopic=226505&view=findpost&p=1391640

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue