mini-os: Make x86_64 support work again
mini-os: Fixed "make clean"

Was failing to delete object files from some subdirectories.
mini-os: Removed test_xenbus from idle thread

Looks like this was just left-over debug code.
mini-os: Added install-ocamlfind target

This installs libminios.a and the linker script to the local ocamlfind
repository, where Mirage can find it.
mini-os: Reorganised arch headers

Before, we had:

- include
- include/arm
- include/x86

Where either arm or x86 was added directly to the search path. So
<x86/arch_limits.h> was the same file as <arch_limits.h> on x86.

We now have:

- include
- arch/arm/include
- arch/x86/include

This means that only the second form is now legal. It also simplifies
installation of the headers, since we can now install everything in the
include directory without getting files for other archs.
mini-os: Removed unused (and missing) header
mini-os: Fixed #ifdef test
mini-os: Made off_t type signed

POSIX requires this.