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 issue 6644.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/18740044