cmd/dist: use -pipe during bootstrap

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/7025044

src/cmd/dist/build.c

@@ -396,6 +396,7 @@ static char *proto_gccargs[] = {
 	"-Werror",
 	"-fno-common",
 	"-ggdb",
+	"-pipe",
 	"-O2",
 };