nbody: discover CUDA_ROOT via 'which nvcc'

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: add debugging options for NVCC

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: fix __CUDA_ARCH__ check for GPU_AOS_tiled

Signed-off-by: Steven Noonan <steven@uplinklabs.net>