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

[Christian Sigg via llvm-commits]

chsigg wrote:

Does this flag correspond to one of the CMake flags? Should it have a similar name, or be referenced in a comment?

I assume this flag will not be tested in CI. Have you tested it locally? Does it trigger a dependency on the CUDA toolkit?

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