runtime: do not preempt race calls
In the crash stack trace race cgocall() calls endcgo(),
this means that m->racecall is wrong.
Indeed this can happen is a goroutine is rescheduled to another M
during race call.
Disable preemption for race calls.
Fixes issue 6155.

R=golang-dev, rsc, cshapiro
CC=golang-dev
https://codereview.appspot.com/12866045