Add AIX platform support

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!.

Start by locating the existing Unix/POSIX implementations and their build-tag declarations in the Go package. Compare the relevant platform files and package tests, then add AIX alongside the existing tags and verify that the package tests still pass. Done means AIX selects the intended implementations without changing other platforms.

Written by the indexing model from the issue text.