Fixes jordansissel/fpm#707

With rpms, if `--architecture` and `--rpm-os` are specified and
different than the host, `--target` would be passed to `rpmbuild` twice,
causing an error.

Example: an OS X host (`uname -s` = "Darwin") with x86_64 architecture
building for `--rpm-os linux` and `--architecture noarch`.

This change only adds `--target` to the `rpmbuild` args once, fixing
this specific error.