cmd/ld: fix Linux/ARM build         CL 5823055 removed a line introduced in Linux/ARM cgo support.         Because readsym() now returns nil for "$a", "$d" mapping symbols,         no matter the settings of `needSym', we still have to guard against         them in ldelf().
cmd/ld: fix Linux/ARM build
        CL 5823055 removed a line introduced in Linux/ARM cgo support.
        Because readsym() now returns nil for "$a", "$d" mapping symbols,
        no matter the settings of `needSym', we still have to guard against
        them in ldelf().

R=golang-dev, dave, rsc
CC=golang-dev
http://codereview.appspot.com/6220073

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