Fixes freebsd/arm build.

Change-Id: Id9b1905a5335f86d317dab7514e0ce7cb74aba1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/193537
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>