[llvm-exegesis] Add CLI Option to set Fixed RNG seed
The primary motivation for this is to set a fixed RNG seed for flaky
tests. This also has the bonus of adding debug logging for what seed
gets used which can make it much easier to reproduce issues that only
happen occasionally and are seed-dependent.
Reviewers: sjoerdmeijer, davemgreen, mshockwave
Reviewed By: davemgreen
Pull Request: https://github.com/llvm/llvm-project/pull/170013
