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 #723.

R=rsc
CC=golang-dev
https://golang.org/cl/1073041

src/Make.pkg

@@ -106,7 +106,7 @@ dir:
 #	x.cgo2.c	- C implementations compiled with gcc to create a dynamic library
 #
 
-_cgo_defun.c _cgo_gotypes.go: $(CGOFILES)
+_cgo_defun.c _cgo_gotypes.go _cgo_export.c _cgo_export.h: $(CGOFILES)
 	CGOPKGPATH=$(dir) $(QUOTED_GOBIN)/cgo $(CGO_CFLAGS) $(CGOFILES)
 
 # Ugly but necessary