cmd/cgo, cmd/go: remove #cgo directive parsing from cmd/cgo

This change removes processing of #cgo directives from cmd/cgo,
pushing the onus back on cmd/go to pass all necessary flags.

Fixes issue 5224. See comments for rationale.

R=golang-dev, iant, r
CC=golang-dev
https://codereview.appspot.com/8610044

Committer: Rob Pike <r@golang.org>