[llvm] [bazel] Add a bazel flag to enable building MLIR with CUDA support (PR #88856)

[Christian Sigg via llvm-commits]

chsigg wrote:

Thanks Adam.

Ok, it seems that `if_cuda_available` doesn't trigger `MLIR_ENABLE_NVPTXCOMPILER` (yet), which would [require](https://github.com/llvm/llvm-project/blob/c88b84d467a201129e329b0bca3425fef326db03/mlir/lib/Target/LLVM/CMakeLists.txt#L52) a CUDA toolkit to be available.

What about converting to NVVM binary though, doesn't [this](https://github.com/llvm/llvm-project/blob/bd84f5d5d71ee26d9552a9cd96ef058cfb8a39fc/mlir/lib/Target/LLVM/NVVM/Target.cpp#L582) call expect to find CUDA tools? 



https://github.com/llvm/llvm-project/pull/88856