runtime/cgo: enable warnings and treat as errors
runtime/cgo: enable warnings and treat as errors

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
http://codereview.appspot.com/6852055

tv42/old-go,dsriram/golang,tv42/old-go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,gnanderson/go,dsriram/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme