[llvm] [mlir][bazel] Define MLIR_CUDA_CONVERSIONS_ENABLED in GPU targets. (PR #83004)

[Oleksandr Alex Zinenko via llvm-commits]

https://github.com/ftynse approved this pull request.

As an FYI, the code relying on this does specifically: [`#if MLIR_CUDA_CONVERSIONS_ENABLED == 1`](https://github.com/llvm/llvm-project/blob/f2bb6c4415954535b32780cd5fb48411ebe0042c/mlir/lib/Target/LLVM/NVVM/Target.cpp#L159) so you may want to define the expected value here, or update that code to use an `#ifdef` instead.

https://github.com/llvm/llvm-project/pull/83004