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 <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

cloudflare/go,golang/go,CAFxX/go,nawawi/go,josharian/go,golang/go,Cofyc/go,codestation/go,golang/go,Cofyc/go,josharian/go,Cofyc/go,codestation/go,golang/go,cloudflare/go,golang/go,josharian/go,akutz/go,akutz/go,cloudflare/go,CAFxX/go,josharian/go,golang/go,akutz/go,cloudflare/go,Cofyc/go,christopher-henderson/Go,codestation/go,Cofyc/go,codestation/go,CAFxX/go,nawawi/go,akutz/go,josharian/go,cloudflare/go,josharian/go,nawawi/go,CAFxX/go,nawawi/go,Cofyc/go,codestation/go,CAFxX/go,josharian/go,CAFxX/go,nawawi/go,codestation/go,Cofyc/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go,akutz/go,golang/go,Cofyc/go,josharian/go,akutz/go,CAFxX/go,akutz/go,codestation/go,nawawi/go,codestation/go,cloudflare/go