xen/cpu: Fix ARM build following c/s 597fbb8

c/s 597fbb8 "xen/timers: Fix memory leak with cpu unplug/plug" broke the ARM
build by being the first patch to add park_offline_cpus to common code.

While it is currently specific to Intel hardware (for reasons of being able to
handle machine check exceptions without an immediate system reset), it isn't
inherently architecture specific, so define it to be false on ARM for now.

Add a comment in both smp.h headers explaining the intended behaviour of the
option.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

xen/include/asm-arm/smp.h

@@ -14,6 +14,12 @@ DECLARE_PER_CPU(cpumask_var_t, cpu_core_mask);
 
 #define raw_smp_processor_id() (get_processor_id())
 
+/*
+ * Do we, for platform reasons, need to actually keep CPUs online when we
+ * would otherwise prefer them to be off?
+ */
+#define park_offline_cpus false
+
 extern void noreturn stop_cpu(void);
 
 extern int arch_smp_init(void);

xen/include/asm-x86/smp.h

@@ -26,6 +26,10 @@ DECLARE_PER_CPU(cpumask_var_t, cpu_sibling_mask);
 DECLARE_PER_CPU(cpumask_var_t, cpu_core_mask);
 DECLARE_PER_CPU(cpumask_var_t, scratch_cpumask);
 
+/*
+ * Do we, for platform reasons, need to actually keep CPUs online when we
+ * would otherwise prefer them to be off?
+ */
 extern bool park_offline_cpus;
 
 void smp_send_nmi_allbutself(void);