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

Updates: #25893

Change-Id: I5c3a9d4403ca170a7e894e06e68b83387d09b816
Reviewed-on: https://go-review.googlesource.com/c/138718
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>