xen64: implement 64-bit Xen support

Hi Ingo,

Here's the set of patches to implement 64-bit Xen support.

The first part of the series is some more (fairly minor) x86 arch
updates which here missed in the previous series.

Following that are the Xen-specific changes to implement 64-bit
support.  It works fairly well, but I know of a couple of bugs:

 - 32-bit emulation doesn't work properly yet.  Something goes wrong
   with %gs, and a userspace %gs: reference segfaults.

 - It crashes when bringing up secondary CPUs under some combinations
   of config.  I think it isn't quite setting up all the CPU sibling
   topology stuff for the various scheduler policies.  It's trying to
   set up the most simple of arrangements (every CPU is a singleton
   with no shared cache or anything else).  It was quite tricky to
   arrange this...

I hope to have followup patches to address both of these in the next
couple of days.  I expect both fixes will be small.

Thanks,
	  J