Enable warnings as errors during the cgo portion of runtime/cgo. iant requested that the list of flags match cmd/dist/build.c, but I would like to avoid the set of disabled warnings if possible.

ref: https://groups.google.com/d/topic/golang-nuts/TrCoVzIIG4M/discussion

requires: 6843061

R=minux.ma, iant
CC=golang-dev
https://golang.org/cl/6852055