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
Edited by Benjamin Schubert