[FR] Add a BUILD file for compare.py

Is your feature request related to a problem? Please describe. Currently, I cannot use compare.py using bazel. The use case scenario is when another repo depends on this repo (i.e. Google benchmark) by adding [git_repository](https://docs.bazel.build/versions/master/repo/git.html#git_repository) to its workspace which references Google benchmark's repo.

Bazel can compile and provides the .hfiles of Google benchmark's repo to the other repo, but currently it cannot do this for compare.py. It would be nice if one could use compare.py from another repo that references Google Benchmark using using git_repository through bazel.

Describe the solution you'd like In my own repo, I can currently call:

bazel run --cxxopt='-std=c++17'  benchmark_sort -c opt

I would like to be able to call something like this:

bazel run --cxxopt='-std=c++17'  benchmark_compare -c opt

where this command calls the compare.py from google benchmark repo.

Describe alternatives you've considered Copying /tool folder to my own repo. This is not ideal cause currently, I don't have to copy the rest of the repo. Bazel automatically does this using git_repository rule in WORKSPACE file.

Additional context