xen: support sysenter/sysexit if hypervisor does

64-bit Xen supports sysenter for 32-bit guests, so support its
use.  (sysenter is faster than int $0x80 in 32-on-64.)

sysexit is still not supported, so we fake it up using iret.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/xen/smp.c

@@ -72,6 +72,7 @@ static __cpuinit void cpu_bringup_and_idle(void)
 	int cpu = smp_processor_id();
 
 	cpu_init();
+	xen_enable_sysenter();
 
 	preempt_disable();
 	per_cpu(cpu_state, cpu) = CPU_ONLINE;