Wrap all of x.py in if __name__ == '__main__': to avoid problems with multiprocessing

[Zalathar]

#110427 caused bootstrap.py to start using the multiprocessing module, which unfortunately broke the bootstrap script on some systems.

It seems that in some situations, Python's multiprocessing module needs to be able to safely import a program's entry-point module without executing the program.

Fixes #111046.

[rustbot]

r? @Mark-Simulacrum

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[Zalathar]

The easier fix would be to just revert #110427, but filing this PR was simple enough that I figured I'd give the option of doing this instead.

[albertlarsan68]

If you could revert the PR in addition of your changes, it would be great.

[jyn514]

Why? This fixes the bug, doesn't it? I don't think we gain anything by reverting

[albertlarsan68]

I (at least) would like to join the pool before continuing and use the os.get_schedaffinity() function instead of the number of CPUs.
If this could be done in this PR, then no need to revert.
The first is especially important, to avoid trying to use the files before they are extracted (may happen with slower CPUs).

EDIT: Per the multiprocessing documentation, the function to use would be len(os.sched_getaffinity(0))

[Zalathar]

I'm not interested in fighting over this.