gh-89792: Limit test_tools freeze test build parallelism based on the number of cores (GH-101841)

unhardcode freeze test build parallelism. base it on the number of cpus, don't use more than max(2, os.cpu_count()/3).
(cherry picked from commit dfc2e065a2e71011017077e549cd2f9bf4944c54)

Co-authored-by: Gregory P. Smith <greg@krypto.org>