path/filepath: add AIX operating system

This commit adds AIX operating system to path/filepath package
for ppc64 architecture.

Updates: #25893

Change-Id: Iea551236d334705a58a446c8992b20adcfe4159b
Reviewed-on: https://go-review.googlesource.com/c/138726Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/path/filepath/path_unix.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 js,wasm linux nacl netbsd openbsd solaris
+// +build aix darwin dragonfly freebsd js,wasm linux nacl netbsd openbsd solaris
 
 package filepath