// dropm is called when a cgo callback has called needm but is now
// done with the callback and returning back into the non-Go thread.
//
// The main expense here is the call to signalstack to release the
// m's signal stack, and then the call to needm on the next callback
// from this thread. It is tempting to try to save the m for next time,
// which would eliminate both these costs, but there might not be