This fixes the Plan 9 support for the new timer code.

Updates #6239
Updates #27707

Change-Id: Ia498c399b8924910b97fcde07545fae3588aad47
Reviewed-on: https://go-review.googlesource.com/c/go/+/204045Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>