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
https://golang.org/cl/6852055

src/pkg/runtime/cgo/cgo.go

@@ -18,6 +18,8 @@ package cgo
 #cgo openbsd LDFLAGS: -lpthread
 #cgo windows LDFLAGS: -lm -mthreads
 
+#cgo CFLAGS: -Wall -Werror
+
 */
 import "C"