C++ modules support

[yuriivcs]

Add Bazel 9 C++ modules configuration for Linux x86_64, Linux aarch64, and cross-compiling between the two.

[ybaturina]

LGTM, but please wait for Richard's review.

[vam-google]

Please check this with entire stack of procjects we support, including TF. If it works on all of them, then LGTM

[rickeylev]

I don't know the cc toolchain configuration internals that well enough to functionally review this sufficiently, but nothing catches my eye as odd or surprising, so LGTM

[rickeylev]

Ah, er -- this sets the default bazel version to 9.1
Is rules_ml_toolchain being tested with other bazel versions somewhere? IIRC, most of the dependent projects are still on bazel 8

[yuriivcs]

Ah, er -- this sets the default bazel version to 9.1 Is rules_ml_toolchain being tested with other bazel versions somewhere? IIRC, most of the dependent projects are still on bazel 8

That’s a good question. Right now, we still manually test rules_ml_toolchain against XLA, JAX, and TF before merging, though we should definitely automate this eventually. The internal toolchain tests only cover the basics, I don't think adding tests for different Bazel versions would add much value.
Thank you