cgo: put temporary source files in _obj.

Fixes #1572.
Initially I tried changing things so all object
files get put in _obj, but it's too much - everything
needs changing. Perhaps some other time.

R=rsc
CC=golang-dev
https://golang.org/cl/4237050

src/cmd/cgo/main.go

@@ -215,6 +215,10 @@ func main() {
 		fs[i] = f
 	}
 
+	// make sure that _obj directory exists, so that we can write
+	// all the output files there.
+	os.Mkdir("_obj", 0777)
+
 	for i, input := range goFiles {
 		f := fs[i]
 		p.Translate(f)