Russ Cox fixed this issue for other systems
in CL 12026, but the Plan 9 part was forgotten.

Fixes #11656.

Change-Id: I91c033687987ba43d13ad8f42e3fe4c7a78e6075
Reviewed-on: https://go-review.googlesource.com/12762Reviewed-by: Russ Cox <rsc@golang.org>