runtime: set up g early

runtime: set up g early

    runtime.throw needs a g to work properly. Set up g early, to
    ensure that if something goes wrong in the runtime startup (e.g.
    runtime.check fails), the program terminates in a reasonable way.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/176657

From-SVN: r271088