cmd/go: fix build -n output when using swig

$INTBITS will not be defined, of course, but that's the best we can do.

Fixes #5978.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13253048

src/cmd/go/build.go

@@ -2162,6 +2162,9 @@ const i int = 1 << 32
 // Determine the size of int on the target system for the -intgosize option
 // of swig >= 2.0.9
 func (b *builder) swigIntSize(obj string) (intsize string, err error) {
+	if buildN {
+		return "$INTBITS", nil
+	}
 	src := filepath.Join(b.work, "swig_intsize.go")
 	if err = ioutil.WriteFile(src, []byte(swigIntSizeCode), 0644); err != nil {
 		return