Summary:
Make `commit_prereq` work and a few other improvements:
* Remove gcc 481 and gcc5xx which are no longer supported
* Remove platform007 which is gone
* `make clean` work for both mac and linux
* `precommit_checker.py` to python3
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9797
Test Plan: `make commit_prereq`
Reviewed By: ajkr
Differential Revision: D35338536
Pulled By: jay-zhuang
fbshipit-source-id: 1e159962ab9d31c43c4b85de7d0f582d3e881ffe