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>