cgo, runtime: diagnose callback on non-Go thread

cgo, runtime: diagnose callback on non-Go thread

Before:
$ go run x.go
signal 11 (core dumped)
$

After:
$ go run x.go
runtime: cgo callback on thread not created by Go.
signal 11 (core dumped)
$

For issue 3068.
Not a fix, but as much of a fix as we can do before Go 1.

R=golang-dev, rogpeppe, gri
CC=golang-dev
https://golang.org/cl/5781047