runtime: allow traceback to trace to caller of invalid onM

runtime: allow traceback to trace to caller of invalid onM onM is the top-of-stack frame on the g0 stack: if the traceback sees it, it assumes we're reached the top of the stack and do not need to trace back any further. Turn calls that did not switch to the g0 stack into jumps, so that the traceback does not see onM and therefore does not stop. I hope this will let me see what is calling onM on the signal stack on the freebsd/amd64 builder. Right now it just shows throw called from badonm called from onM and then stops.