lib9/fmt: do not put a main symbol into lib9.a
lib9/fmt: do not put a main symbol into lib9.a

Fixes issue 4143.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6541064

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