[xen]: pass '-linkpkg -dontlink unix' to ocamlfind

When using later OCaml versions we don't emit a myocamlbuild,
so we can't rely on rules there.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Revert "[xen] add a -dontlink unix flag"

This reverts commit 2c04389d03b04aa4b02e31a85e0d2198a3e9c2cc.
Revert "[xen] fix the output_obj to include linkpkg"

This reverts commit a7b5014e66c17089a69e3b93a8205577d5e77e34.