feat: Migrate to bzlmod

[blorente]

Migrate gcc-toolchain to a bzlmod package.

This change intentionally doesn't:

• Update the documentation to reflect that bzlmod can be used now. This should be done when gcc-toolchain exists in the BCR.
• Default to bzlmod in the .bazelrc file. Instead, it creates a configuration bzlmod that enables it. I think we should add bzlmod to the test matrix, but I'd like @f0rmiga 's guidance on whether to add it for all configurations, or just for some.

I'm happy to change any of the above.

Testing:

Tested by running bazel test --config=bzlmod //... on an x86_64 machine running pretty stock Ubuntu 22.04. All tests pass.

Also tested by cross-compiling a target to aarch64 in a project that sets --noenable_workspace.

[f0rmiga]

For CI, we should do a matrix and run all pipelines with bzlmod and workspaces.

[blorente]

For CI, we should do a matrix and run all pipelines with bzlmod and workspaces.

@f0rmiga Thanks for the clarification! CI changes made here: fd97819