[IA64] Kexec: partial port of CPU_HOTPLUG

[IA64] Kexec: partial port of CPU_HOTPLUG

* Enable CONFIG_CPU_HOTPLUG

* Add #ifndef CONFIG_XEN as appropriate around portions that are not
  needed for kexec - it is used to take down cpus on SMP systems
  before kexecing.

* Port various xen-specific bits as neccessary
  - This has mainly been done in the existing kexec-related files,
    as kexex is currently the only user of this code. If a full
    port of CPU_HOTPLUG was done then this code would either disapear
    or be relocated elsewhere.

Signed-off-by: Simon Horman <horms@verge.net.au>