These are already exposed on Linux. I believe they were left out of
darwin because fchmodat didn't exist on older versions of XNU.

Change-Id: I166f2b23270937c1b6cc3bd73e7f7b72d2d488e9
Reviewed-on: https://go-review.googlesource.com/14092Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>