Fixes#95.
CPU-only mode is enabled by setting the `is_cpu_only` property while
defining a benchmark, e.g. `NVBENCH_BENCH(foo).set_is_cpu_only(true)`.
An optional `nvbench::exec_tag::no_gpu` hint can also be passed to
`state.exec` to avoid instantiating GPU benchmarking backends. Note that
a CUDA compiler and CUDA runtime are always required, even if all benchmarks
in a translation unit are CPU-only.
Similarly, a new `nvbench::exec_tag::gpu` hint can be used to avoid
compiling CPU-only backends for GPU benchmarks.
