Make the bazel command and threads count configurable

[sthornington]

This is a quick thing we needed to do in our environment to get everything working.

1. Make the bazel command configurable - we need to call bazel from a wrapper script to set up some stuff, and as a consequence, the command that this tool subshells needs to use that too. I made this as a --define command line parameter so that it could be passed from the original invocation of bazel when running the tool.

2. Make the max thread count configurable. As a consequence of the above, we don't want many instances of our bazel wrapper running concurrently, that can cause spurious bazel dump --action_cache failures, so I added a rule parameter so we can just hard code it in our rule wrapper.

3. Almost forgot, I switched print_args from iostream to ANSI C using printf, so that building that doesn't have to know where/which C++ runtime etc to use.

This is my first time doing bazel so if these are the wrong approaches please let me know. Thanks!

[sthornington]

In my private fork I have added:

1. change print_args to use stdio instead of iostreams
2. add max_threads rule parameter
3. added BAZEL_COMMAND --define parameter
4. switch to ld.gold so that building print_args doesn't barf on --threads
5. new file extension exclusion list param to avoid using codegen files to compile headers in clang-tidy

Happy to update the PR with all of these if someone ever wants.