dragonfly/386 isn't a valid GOOS/GOARCH pair.

Change-Id: I44374a932b20f0d49b2e509484143970eb5464c2
Reviewed-on: https://go-review.googlesource.com/79656
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>