Add _cgo_export.[ch] targets so that make knows how they are built.  Note that they are only built if a //export comment appears, so other targets should only use them if there is such a comment.
Add _cgo_export.[ch] targets so that make knows how they are
built.  Note that they are only built if a //export comment
appears, so other targets should only use them if there is
such a comment.

Fixes issue 723.

R=rsc
CC=golang-dev
http://codereview.appspot.com/1073041

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