Last of the Macos libSystem changes, hopefully.

Fixes #17490

Change-Id: I88b303bafd92494cc4ddde712213d2ef976ce4e2
Reviewed-on: https://go-review.googlesource.com/c/155737
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>