go/build already ignores them, but they cause make.bash to fail.

Fixes #18931.

Change-Id: Idd5c8c2a6f2309ecd5f0d669660704d6f5612710
Reviewed-on: https://go-review.googlesource.com/36351
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>