runtime: allow copying of onM frame
Currently goroutines in onM can't be copied/shrunk
(including the very goroutine that triggers GC).
Special case onM to allow copying.

LGTM=daniel.morsing, khr
R=golang-codereviews, daniel.morsing, khr, rsc
CC=golang-codereviews, rlh
https://codereview.appspot.com/124550043