.network/.netdev/.link: allow to match on architecture
build-sys: make network files respect --rootprefix

With --rootprefix= systemd-udevd gets installed to /lib/systemd, and since
the network configuration is also required during early boot, it should be
available there with it. Using --prefix= is not an option since it would
put everything, including pkg-config files, man pages, documentation, to /
which is not wanted. This commit puts 99-default.link to
/lib/systemd/network/ when required.