mime: add AIX operating system

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

Updates: #25893

Change-Id: I1b81a9204446c8c76ae24ea5c6fd33077c373ae4
Reviewed-on: https://go-review.googlesource.com/c/138723
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/type_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 mime