Refactor repository folders

[ybaturina]

1. Move common tools to //common.
2. Move all GPU-related repo rules to //gpu
3. Move extensions for bzlmode to //extensions.
4. Remove unused Mac and Windows toolchains from CUDA config.

[yuriivcs]

Suggestions (not mandatory to do anything from below list):
Would it be possible to rename //gpu/cuda/crosstool to //gpu/cuda/legacy (or move it there)? This would help developers who work with rules_ml_toolchain understand that this code is already deprecated (some developers still think that this is mainstream toolchains).
Additionally, I think we could simplify gpu/cuda/local_config_cuda.BUILD by updating the deprecated settings in our ML projects to point to the configuration in //common and then remove most of configuration from this file.

[yuriivcs]

LGTM