Add support for OCaml 4.02+ in Xen

This releases adds support for OCaml 4.02+ compilation, and changes the Xen
backend build for Mirage significantly by:

• removing the OCaml compiler runtime from the mirage-platform, which makes
  it simpler to work across multiple revisions of the compiler. It now uses
  the ocaml-src OPAM package to grab the current switch's version of the
  OCaml runtime.
• split the Xen runtime build into discrete pkg-config libraries:
  • mirage-xen-posix.pc : in the xen-posix/ directory, is the nano-posix
    layer built with no knowledge of OCaml
  • mirage-xen-minios.pc: defines the __INSIDE_MINIOS__ macro to expose
    internal state via the MiniOS headers (for use only by libraries that
    know exactly what they are doing with the MiniOS)
  • mirage-xen-ocaml.pc: in xen-ocaml/core/, this builds the OCaml asmrun,
    Bigarray and Str bindings using the mirage-xen-posix layer.
  • mirage-xen-ocaml-bindings.pc: in xen-ocaml/bindings/, these are bindings
    required by the OCaml libraries to MiniOS. Some of the bindings use MiniOS
    external state and hence use mirage-xen-minios, whereas others
    (cstruct_stubs and barrier_stubs are just OCaml bindings and so just
    use mirage-xen-posix).
  • mirage-xen.pc: depends on all the above to provide the same external
    interface as the current mirage-platform.

The OCaml code is now built using OASIS, since the C code is built entirely
separately and could be moved out into a separate OPAM package entirely.