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

Updates: #25893

Change-Id: I46bbc7b03931019beb969443cb3f9a756956c66c
Reviewed-on: https://go-review.googlesource.com/c/138724
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>