libxl: ocaml: guard x86-specific functions behind an ifdef

The various cpuid functions are not available on ARM, so this
makes them raise an OCaml exception.  Omitting the functions
completely results in a link failure in oxenstored due to the
missing symbols, so this is preferable to the much bigger patch
that would result from adding conditional compilation into the
OCaml interfaces.

With this patch, oxenstored can successfully start a domain on
Xen/ARM.

Signed-off-by: Anil Madhavapeddy <anil@recoil.org>
Acked-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>