gc: omit runtime.closure wrap for closures without closure variables
gc: omit runtime.closure wrap for closures without closure variables

Fixes issue 1894.

test/closure.go's test for newfunc already covers this.

R=rsc, dsymonds, bradfitz
CC=golang-dev
http://codereview.appspot.com/5516051

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