Add support for nvcc & hipcc (cuda/rocm) (#23805)

I've been working on making some basic cuda examples work, both with
cuda (nvcc) and with AMD HIP (hipcc) https://github.com/monofuel/hippo

- hipcc is just a drop-in replacement for clang and works out of the box
with clang settings in Nim. hipcc is capable of compiling for AMD ROCm
or to CUDA, depending on how HIP_PLATFORM is set.
- nvcc is a little quirky. we can use `-x cu` to tell it to handle nim's
`.cpp` files as if they were `.cu` files. nvcc expects all backend
compiler flags to be wrapped with a special `-Xcompiler=""` flag when
compiling and also when linking.

I manually tested on a linux desktop with amd and a laptop with nvidia.

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