runtime: don't terminate locked OS threads on Plan 9

CL 46037 and CL 46038 implemented termination of
locked OS threads when the goroutine exits.

However, this behavior leads to crashes of Go programs
using runtime.LockOSThread on Plan 9. This is notably
the case of the os/exec and net packages.

This change disables termination of locked OS threads
on Plan 9.

Updates #22227.

Change-Id: If9fa241bff1c0b68e7e9e321e06e5203b3923212
Reviewed-on: https://go-review.googlesource.com/71230Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/proc.go

@@ -2671,8 +2671,10 @@ func goexit0(gp *g) {
 
 		// Return to mstart, which will release the P and exit
 		// the thread.
+		if GOOS != "plan9" { // See golang.org/issue/22227.
 			gogo(&_g_.m.g0.sched)
 		}
+	}
 	schedule()
 }