os: fix readdirnames for AIX

Largefile implementation should be used on AIX.

readdir64_r function returns 9 and sets result to NULL when
reaching end of directory, so this return code should not
always be considered as an error.

Issue golang/go#19200

Change-Id: I86e6944d1a9b6c80ea166cfa142a92d911e4be8f
Reviewed-on: https://go-review.googlesource.com/38359
Reviewed-by: Ian Lance Taylor <iant@golang.org>
3 files changed