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

Follows CL 79675 which did the same for syscall.

Change-Id: Ie70937b464fa911a43c26515bcac1c03740dc1fe
Reviewed-on: https://go-review.googlesource.com/79676Reviewed-by: Ian Lance Taylor <iant@golang.org>