This commit adds AIX operating system to internal/poll package for ppc64
architecture.

Updates: #25893

Change-Id: I9b1da9255012de58f16547c1b18f8840485da170
Reviewed-on: https://go-review.googlesource.com/c/138717
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>