cmd/ld: don't show cursym in gcc invocation failure message
before:
fmt.statictmp_0731:/usr/local/go/pkg/tool/linux_amd64/6l: running gcc failed: unsuccessful exit status 0x100
now:
/usr/local/go/pkg/tool/linux_amd64/6l: running gcc failed: unsuccessful exit status 0x100

R=golang-dev, iant, r
CC=golang-dev
https://codereview.appspot.com/8827045