compiler, runtime: better stack trace for `go f()` where f is nil

compiler, runtime: better stack trace for `go f()` where f is nil

    The test for this is TestGoNil in the runtime package, which we don't
    run yet but will run with a subsequent gotools patch.

    Updates golang/go#8045

    Reviewed-on: https://go-review.googlesource.com/46392

From-SVN: r249494