internal/syscall: add AIX operating system

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>

src/internal/syscall/unix/nonblocking.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package unix