runtime: enable preemption of mark termination goroutine
A mark worker goroutine may attempt to preempt the mark termination
goroutine to scan its stack while the mark termination goroutine is
trying to preempt that worker to flush its work buffer, in rare
cases.
This change makes it so that, like a worker goroutine, the mark
termination goroutine stack is preemptible while it is on the
system stack, attempting to preempt others.