cmd/ld: fix -s flag for ELF executables
cmd/ld: fix -s flag for ELF executables

This fixes a regression introduced in changeset 98034d036d03
which added support for producing host object files.

R=rsc, minux.ma
CC=dave, golang-dev
https://codereview.appspot.com/7307107
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go