[release-branch.go1.4] runtime: fix deadlock in runtime.Stack

It shouldn't semacquire() inside an acquirem(), the runtime
thinks that means deadlock.  It actually isn't a deadlock, but it
looks like it because acquirem() does m.locks++.

Candidate for inclusion in 1.4.1.  runtime.Stack with all=true
is pretty unuseable in GOMAXPROCS>1 environment.

fixes #9321

Change-Id: Iac6b664217d24763b9878c20e49229a1ecffc805
Reviewed-on: https://go-review.googlesource.com/1600


Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
(cherry picked from commit 50bc3d5b)
Reviewed-on: https://go-review.googlesource.com/2807


Reviewed-by: Andrew Gerrand <adg@golang.org>