cmd/cgo: omit //line in -godefs, -cdefs output

Makes files like src/pkg/syscall/ztypes_linux_amd64.go easier to read.
(The copy that is checked in predates the //line output mode,
so this also preserves the status quo.)

R=golang-dev, iant, gri
CC=golang-dev
https://golang.org/cl/5655068

src/cmd/cgo/main.go

@@ -15,6 +15,7 @@ import (
 	"flag"
 	"fmt"
 	"go/ast"
+	"go/printer"
 	"go/token"
 	"io"
 	"os"
@@ -158,6 +159,13 @@ func main() {
 		os.Exit(2)
 	}
 
+	if *godefs || *cdefs {
+		// Generating definitions pulled from header files,
+		// to be checked into Go repositories.
+		// Line numbers are just noise.
+		conf.Mode &^= printer.SourcePos
+	}
+
 	args := flag.Args()
 	if len(args) < 1 {
 		usage()