gh-76327: freeze_support for non-Windows platforms

[zetaloop]

gh-76327: freeze_support for non-Windows platforms

This is a brief fix for the multiprocessing frozen executable infinite loop issue on non-Windows platform. #76327
And the freeze_support patch was exactly the same patch from PyInstaller, which already worked for them.

This patch does not influence PyInstaller.
However, this patch will enable cx_freeze or other similar tools to build multiprocessing apps for Linux and Mac.

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📚 Documentation preview 📚: https://cpython-previews--104607.org.readthedocs.build/

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[zetaloop]

Backport needed.

[zetaloop]

Tests done.
Windows 11 (Python 3.11.1): PyInstaller✔️ cx_freeze✔️ py2exe✔️
WSL Ubuntu 22.04 (Python 3.10.6): PyInstaller✔️ cx_freeze✔️

Note: if we don't modify muliprocessing, cx_freeze on Linux will create an app with terrible infinite loop.

import multiprocessing

def worker():
    print('Worker')

if __name__ == '__main__':
    multiprocessing.freeze_support()
    multiprocessing.set_start_method('spawn')
    p = multiprocessing.Process(target=worker)
    p.start()