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.

Fixes #28695.

Change-Id: I23bbb191f4fdad293e8a70befd51c9175f8a1171
Reviewed-on: https://go-review.googlesource.com/c/153077


Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Austin Clements <austin@google.com>