x86/xen: Consolidate xen-os.h in a single place

amd64 and i386 platform code contain very similar xen/xen-os.h

The only differences are:

• Functions/variables/types which were unused in i386/xen/xen-os.h:
  • xen_xchg
  • __xchg_dummy
  • __xg
  • __xchg
  • atomic_t
  • atomic_inc
  • rdtscll

The functions/variables/types unused in xen-os.h can be dropped and there
is no more differences betwen amd64 and i386.

The new header is placed in x86/include/xen and each platform will have
dummy headers include x86/xen/*.h. This is to be able to include
machine/xen/*.h in the PV drivers.

Submitted by: Julien Grall <julien.grall@citrix.com>
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D3880
Sponsored by: Citrix Systems R&D