runtime: reap idle Ms

[randall77]

Ms cost memory (stacks, mostly, maybe kernel resources also) to keep around. See issue #22439. It would be better if we got rid of Ms if they aren't being used. Maybe exit Ms that have been idle for a while during GC? During scavenging? Maybe just exit idle Ms unconditionally at scavenge time, we'll make new ones if we need them.

@aclements recently added mechanisms to reap Ms, for exiting LockOSThread-ed Ms. We can reuse that mechanism.

[aclements]

Closing as dup of #14592.