Extricate rumprun build system from Mini-OS

Moves the Mini-OS Makefile and associated bits to xen/, separating the
rumprun and Mini-OS build systems. This will hopefully prepare us better
for (possibly) merging with upstream Mini-OS.

Further changes arising from this:

- All minios and rumprun object files are now built under obj/

- MiniOS build installs the arch-specific ldscript under
  obj/xen/minios.lds

- Rather than separately specifying the arch-specific startfile in
  app-tools specs we link it into the final minios.o and use that as the
  startfile.

  The advantage of this approach is we no longer have to "hunt" for the
  startfile _OR_ the arch-specific lib in the top-level Makefile. It
  does mean the resulting application has a different memory layout due
  to link order but this should not matter.

Signed-off-by: Martin Lucina <martin@lucina.net>
Mini-OS: Add monotonic_clock to minios_ namespace

This is needed by rumphyper_base, among other things. Unclear how it is
possible that the build was working until now, separating the Mini-OS
and rumprun build systems exposed this.

Signed-off-by: Martin Lucina <martin@lucina.net>