dragonfly/386 isn't a valid GOOS/GOARCH pair and there are no generated
files for this pair in syscall.

Change-Id: Ibea2103c2f5e139139d850df3aac9b5a9c4ac9ab
Reviewed-on: https://go-review.googlesource.com/79675Reviewed-by: Ian Lance Taylor <iant@golang.org>