oasis/mirage pull req

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

All, while the cstruct hacking is ongoing, one of the last remaining 
"usability" items is having good OASIS support for building the Xen backend.

I've split this up into two pieces:

- Support in OASIS for generating a standalone `.o` object file that contains 
the linked OCaml application, but not the runtime.  All the phases after this 
do not need the OCaml compiler any more.

- The Xen microkernel link phase that uses a custom lds script to convert the 
.o file into a standalone Xen binary.

I've sent a pull request upstream to the OASIS developers for the first item:
https://github.com/ocaml/oasis/pull/7

This adds a `.nobj.o` output target to OASIS, and will be sufficient for us for 
now (including for Haris' NS3 backend, and Gabor's FreeBSD backend).

-anil