They do not match the file name patterns of
  *_GOOS
  *_GOARCH
  *_GOOS_GOARCH
therefore the implicit linux constraint was not being added.

Change-Id: Ie506c51cee6818db445516f96fffaa351df62cf5
Reviewed-on: https://go-review.googlesource.com/99116Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>