This commit adds AIX operating system to os package for ppc64
architecture.

Updates: #25893

Change-Id: Ieb9a2b3ac5b9abd3b5ab68eb732c17b6256d624d
Reviewed-on: https://go-review.googlesource.com/c/138725Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>