ARM support

This pull request is for tracking the progress of the ARM work. It adjusts the generated Makefile to link against libminios and openlibm (via pkg-config) and to generate an ARM boot executable zImage on ARM platforms. It requires Git versions of several other components:

* https://github.com/talex5/xen/tree/arm-preview (compile Mini-OS as a library + ARM)
* https://github.com/JuliaLang/openlibm (pkg-config + ARM support)
* https://github.com/talex5/mirage-platform/tree/arm-preview (uses above components)