cmd/nm: do not fail TestNM if symbol has less then 3 columns in nm output
cmd/nm: do not fail TestNM if symbol has less then 3 columns in nm output

Fixes issue 7829

LGTM=dave
R=golang-codereviews, aram, dave
CC=golang-codereviews
https://codereview.appspot.com/89830043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang