i386 Xen integration breaks GENERIC_PAE kernel build

["John D. Baker" <jdbaker%consolidated.net@localhost>]

Building the GENERIC_PAE kernel from recent -current/i386 fails with:

[...]
--- hypervisor.o ---
/x/current/src/sys/arch/xen/xen/hypervisor.c: In function 'init_xen_early':
/x/current/src/sys/arch/xen/xen/hypervisor.c:247:27: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
  HYPERVISOR_shared_info = (void *)(HYPERVISOR_shared_info_pa + KERNBASE);
                           ^


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