gh-109649: Use os.process_cpu_count() (#110165)

Replace os.cpu_count() with os.process_cpu_count() in modules:

* compileall
* concurrent.futures
* multiprocessing

Replace os.cpu_count() with os.process_cpu_count() in programs:

* _decimal deccheck.py test
* freeze.py
* multissltests.py
* python -m test (regrtest)
* wasm_build.py

Other changes:

* test.pythoninfo logs os.process_cpu_count().
* regrtest gets os.process_cpu_count() / os.cpu_count() in headers.