Add os.getnumcpus function to get number of logical CPU cores.

[tritao]

What does this PR do?

Adds a new os.getnumcpus API to get the number of logical CPU cores.

This can be helpful to know the parameter to pass to make -j flag, which in my case I need to generate correct VS Code tasks files.

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• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
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• Align documentation to your changes

[tritao]

Thanks for the super fast review, addressed the nits 👍

[tritao]

Updated with new round of reviews.

[nickclark2016]

Pretty minor, fix and I'll merge.

[tritao]

Updated again.

[theComputeKid]

Thanks for this! Previously, I had to use this code for my projects:

function getNumProc()
    if os.host() == "windows" then
        return os.getenv("NUMBER_OF_PROCESSORS")
    else
        output, errorCode = os.outputof("getconf _NPROCESSORS_ONLN")
        return output
    end
end

Now I can use the function you have added instead.