runtime: Plan 9 - prevent preemption by GC while exiting

runtime: Plan 9 - prevent preemption by GC while exiting

On Plan 9, there's no "kill all threads" system call, so exit is done
by sending a "go: exit" note to each OS process.  If concurrent GC
occurs during this loop, deadlock sometimes results.  Prevent this by
incrementing m.locks before sending notes.

Change-Id: I31aa15134ff6e42d9a82f9f8a308620b3ad1b1b1
Reviewed-on: https://go-review.googlesource.com/20477
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>