os: use opendir64 and closedir64 on AIX

When using readdir64 on AIX, we must also use
opendir64 and closedir64.

Issue golang/go#19200

Change-Id: I14211a0bba0bf0c848568c4ae9d3821bcbc2c353
Reviewed-on: https://go-review.googlesource.com/43251
Reviewed-by: Ian Lance Taylor <iant@golang.org>
3 files changed