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

apparentlymart/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,dsriram/golang,tv42/old-go,dsriram/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go