Last runtime use was removed in https://golang.org/cl/133700043,
September 2014.

Replace plan9 syscall uses with plan9-specific variable.

Change-Id: Ifb910c021c1419a7c782959f90b054ed600d9e19
Reviewed-on: https://go-review.googlesource.com/55450Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>