LLVM package: disable CUDA (#10858)
LLVM package: disable CUDA (#10858)

This avoids using a system-installed CUDA package. In the future a
variant can be added to allow using Spack-installed CUDA, but for
now CUDA support is always disabled.
iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack