runtime: disable use of template thread on Plan 9

runtime: disable use of template thread on Plan 9

CL 46033 added a "template thread" mechanism to
allow creation of thread with a known-good state
from a thread of unknown state.

However, we are experiencing issues on Plan 9
with programs using the os/exec and net package.
These package are relying on runtime.LockOSThread.

Updates #22227.

Change-Id: I85b71580a41df9fe8b24bd8623c064b6773288b0
Reviewed-on: https://go-review.googlesource.com/70231
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>