Added minimal POSIX API

Just enough to let us compile and run the OCaml runtime. Writing to
stdout or stderr outputs the string with printk, which is useful
for debugging.
Use musl's float-formatting code to handle %f in sprintf
Override start_kernel

This allows us to skip initialisation of Mini-OS's support for
threads and XenBus, which we don't need.
Fixes for compiling on ARM
Hacks for compiling on ARM
Hack to allow building under OPAM

OPAM doesn't preserve symlinks, so readlink doesn't work.
Hack: Replaced Xen checksum code with Unix version

The Xen version does unaligned double-word accesses, which fault on ARM.