Add AIX platform support

[ayappanec]

Hi Team,
I would like to add AIX platform support to this project. The changes required are very minimal and mostly involve adding "aix" build tags alongside the existing Unix/POSIX implementations.
If welcomed, I can submit a PR with the changes!.

[estesp]

I think that's reasonable as long as there is no expectation of support. But I think this project already has build enablement for many platforms not supported by the containerd project (zos, freebsd, netbsd, etc.), so I don't see any reason why we can't add AIX. Feel free to submit a PR. Are you focused only on the console sub-project or is the goal to get containerd itself running on AIX?

[ayappanec]

Thanks.
Right now, the focus is only on the console sub-project.

[samuelkarp]

aix build tags along with a build/compile check seems fine to me. You can see our platform documentation (which is focused on containerd itself, but the concepts apply here) and that this would fit as tier 3.

[ayappanec]

aix build tags along with a build/compile check seems fine to me. You can see our platform documentation (which is focused on containerd itself, but the concepts apply here) and that this would fit as tier 3.

Sounds good. Thanks