libudev-zero's `Makefile` sets `AR=ar`; we must override this when
cross compiling. With this commit,
`pkgsCross.aarch64-multiplatform.libudev-zero` builds.
libudev-zero is a daemonless reimplementation of udev, that is of
additional interest to us in Nixpkgs because (currently with a small
upstreamable build system patch) it's possible to build it statically,
which systemd's udev is unlikely to support in the near future[1].
In future, it might be good to default to using libudev-zero in
pkgsStatic, but for now let's just introduce the package, so people
doing static builds can at least use it themselves using overrides or
overlays.
[1]: https://github.com/systemd/systemd/pull/20621#issuecomment-912014839
