Add CUDA-wrappers submodule to replace util/cu.* and util/nvrtc.*

The https://github.com/nlesc-recruit/CUDA-wrappers repository is added as external/cuda-wrappers git module. The Makefile is updated to run cmake and make for this module and to use the resulting libcu.so for the functionality previously provided by util/cu.* and util/nvrtc.*.