[IA64] sparse tree cleanups

[IA64] sparse tree cleanups

 Here's a first pass at some sparse tree cleanups based on feedback
 we've gotten on linux-ia64.  I haven't taken all of the comments into
 account yet, this is just a start.  Specific changes:

  - CONFIG_ARCH_XEN is no longer necessary
  - simple is_running_on_xen() checks can be handled w/o CONFIG_XEN
  - pulled forward a drivers/xen/Makefile change that broken make clean
  - changed some comments from C++ style to C
  - turned HYPERVISOR_ioremap into a macro for !CONFIG_XEN
  - made hypervisor.h and privop.h include-able for !CONFIG_XEN

Signed-off-by: Alex Williamson <alex.williamson@hp.com>