cmd/go: fix build -n output when using swig
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 issue 5978.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13253048

wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old