This commit add statUnix function for aix/ppc64. It also adds Unix
and Nano methods for AIX time structure.

Change-Id: I9fd62d34a47e87cd46f2f936cb736da0bdff7959
Reviewed-on: https://go-review.googlesource.com/c/163957
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>