Add support for building on FreeBSD

[zebrapurring]

This PR adds support for building the package on FreeBSD through PTHREAD_NP, since it is not compatible with the gettid syscall.

Tested on FreeBSD 13.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c405d15) to head (78b7f20).

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Additional details and impacted files

@@            Coverage Diff            @@
##            master      #432   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           21        21           
  Lines         2308      2308           
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  Hits          2308      2308           

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[gaogaotiantian]

Thanks. We don't have official support for FreeBSD because there's no github action runner for it. However, the patch is easy to follow and does not affect the platforms we do support. I'll merge it for people who use FreeBSD :)

[zebrapurring]

Thanks! Some features like attaching to processes are still not working on FreeBSD, but at least this should allow installing the package via Pip and generating traces.

[gaogaotiantian]

Attaching would be hard. That is some nasty hack feature anyway.