[dev.cc] runtime: generate GOOS- and GOARCH-specific files with go generate

Eventually I'd like almost everything cmd/dist generates
to be done with 'go generate' and checked in, to simplify
the bootstrap process. The only thing cmd/dist really needs
to do is write things like the current experiment info and
the current version.

This is a first step toward that. It replaces the _NaCl etc
constants with generated ones goos_nacl, goos_darwin,
goarch_386, and so on.

LGTM=dave, austin
R=austin, dave, bradfitz
CC=golang-codereviews, iant, r
https://codereview.appspot.com/174290043