// cgocallback is not called from Go, only from crosscall2.
// This in turn calls cgocallbackg, which is where we'll find
// pointer-declared arguments.
-//
-// When fn is nil (frame is saved g), call dropm instead,
-// this is used when the C thread is exiting.
func cgocallback(fn, frame, ctxt uintptr)
func gogo(buf *gobuf)