cmd/cgo: extend implementation comment

This is the plan for how to make host linking work with
the rest of the system.

There are two complications:

1. It is a goal to preserve the property that pure Go programs
(even ones importing "net") can be compiled without needing
gcc, so that a Go toolchain download works out of the box.
This forces the support for two linking modes: with and without
gcc.

2. It is a goal to allow users with old copies of SWIG to continue
to use those copies. This forces the support for "internal only"
packages. Perhaps it is reasonable to require a new SWIG.
I don't know.

R=iant
CC=golang-dev
https://codereview.appspot.com/7433043