cmd/cgo: add AIX operating system

cmd/cgo: add AIX operating system

This commit adds AIX operating system to cmd/cgo package for ppc64
architecture.

It doesn't fully adapt cgo tool to AIX. But it allows to use
go tool cgo -godefs which is really usefull for others packages.

Update: #25893

Change-Id: I38e289cf0122d143ba100986d08229b51b03ddfc
Reviewed-on: https://go-review.googlesource.com/c/138731
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>