xen: Fix bug `do_IRQ: cannot handle IRQ -1 vector 0x6 cpu 1'

Following commit 9c3f2468d8339866d9ef6a25aae31a8909c6be0d, do_IRQ()
looks up the IRQ number in the per-cpu variable vector_irq.

This commit makes Xen initialise an identity vector_irq map for both X86_32 and X86_64.
Signed-off-by: Alex Nixon <alex.nixon@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/irq.c

@@ -21,7 +21,6 @@ void xen_force_evtchn_callback(void)
 
 static void __init __xen_init_IRQ(void)
 {
-#ifdef CONFIG_X86_64
 	int i;
 
 	/* Create identity vector->irq map */
@@ -31,7 +30,6 @@ static void __init __xen_init_IRQ(void)
 		for_each_possible_cpu(cpu)
 			per_cpu(vector_irq, cpu)[i] = i;
 	}
-#endif	/* CONFIG_X86_64 */
 
 	xen_init_IRQ();
 }