runtime: relax preemption assertion during stack split

The case can happen when starttheworld is calling acquirep
to get things moving again and acquirep gets preempted.
The stack trace is in golang.org/issue/6644.

It is difficult to build a short test case for this, but
the person who reported issue 6644 confirms that this
solves the problem.

Fixes #6644.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/18740044

src/pkg/runtime/stack.c

@@ -255,7 +255,7 @@ runtime·newstack(void)
 	if(gp->stackguard0 == (uintptr)StackPreempt) {
 		if(gp == m->g0)
 			runtime·throw("runtime: preempt g0");
-		if(oldstatus == Grunning && m->p == nil)
+		if(oldstatus == Grunning && m->p == nil && m->locks == 0)
 			runtime·throw("runtime: g is running but p is not");
 		if(oldstatus == Gsyscall && m->locks == 0)
 			runtime·throw("runtime: stack split during syscall");