Fix build error on darwin/{arm,arm64} caused by a62ae9f6.

Change-Id: I7da4d147a529b11b3e71352a9276a57595ec4b05
Reviewed-on: https://go-review.googlesource.com/21356
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>