cmd/dist: add AIX operating system.

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

The stack guard is increased because of syscalls made inside the runtime
which need a larger stack.

Disable cmd/vet/all tests until aix/ppc64 is fully available.

Change-Id: I7e3caf86724249ae564a152d90c1cbd4de288814
Reviewed-on: https://go-review.googlesource.com/c/138715


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>