x86/xen: eliminate xen-asm_64.S

With 32-bit pv-guest support removed xen-asm_64.S can be merged with
xen-asm.S

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>