Add linker options for Xen/Mini-OS

[talex5]

Note: depends on mirage/mirage-platform#111

[yallop]

Thanks!

Is there an easy way to set up CI for ctypes on Xen/Mini-OS? It'd be useful to have some kind of automatic check to avoid inadvertently breaking the build.

[talex5]

I've added some tests. Note however that they won't pass until the next mirage-platform release.

[avsm]

mirage-platform releases with this change now in ocaml/opam-repository#3219

[yallop]

The tests are passing on linux/4.01.0 now, but failing on osx (as expected, presumably).

[talex5]

I would have thought you should be able to install mirage-xen on OS X, but I don't have a machine to test it.

[avsm]

mirage-xen wont compile on osx just yet due to the mirage-xen-minios dep

[talex5]

I've updated the tests to check the Xen support only when mirage-xen is installable. This means that when more platforms are supported, the tests should run there too.

[yallop]

Thanks, @talex5!