cmd/go: use new SWIG -cgo option

This fixes SWIG to work again.  It requires SWIG 3.0.6 or later.
Earlier versions of SWIG will not work because they generate a .c file
to be compiled by [568]c, which no longer exist.  As of SWIG 3.0.6
SWIG supports a -cgo option that tells it to generate files that
import "C" and can be used with the cgo tool.  With luck this will
means that future versions of SWIG will not require changes for future
versions of Go.

Change-Id: Iad7beb196ba9dcd3e3f684196d50e5d51ed98204
Reviewed-on: https://go-review.googlesource.com/6851Reviewed-by: Russ Cox <rsc@golang.org>