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 (`uname -m`) architecture building for `--rpm-os linux` and `--architecture noarch`.

This change only adds `--target` to the `rpmbuild` args once, fixing this specific error.