syscall: use internal/byteorder in plan9 directory marshaling helpers

syscall: use internal/byteorder in plan9 directory marshaling helpers

Change-Id: I277565ddb348e6a7cca18015d3d80abf0575b5a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/585716
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>