Get Xen linker options from ocamlfind xen_linkopts variables

Before, we used the normal linker flags, but we want to share compiled
OCaml libraries between Unix and Xen while having separate C stubs.
Accidentally linking in Unix C stubs on Xen is bad on x86_64 because
Unix stubs normally assume the red zone is available. ocamlopt currently
misses this optimisation and therefore OCaml code can be shared. We
might want to revisit this design once OPAM supports cross-compiling.