Don't use psutil.Process.cpu_affinity on platform not supporting it

Summary

Some platforms like AIX/Solaris don't support get_cpu_affinity, however, the base platform assumes every thing not darwin supports it.

This results in an exception being thrown on those platforms when trying to do anything.

This is used at: https://gitlab.com/BuildStream/buildstream/blob/master/src/buildstream/_platform/platform.py#L125

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