x86_64: Missing a volatile on a struct used for spinning

Before we're initialized and can use proper synchronization, the CPU
initialization path spins on the thread entry function to be non-null.
But the data wasn't tagged volatile, and with gcc 8.2.1 (but not
6.2.0) the optimizer was hoisting the reads to SMP init would spin
forever.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

arch/x86_64/core/x86_64.c

@@ -69,7 +69,7 @@ void *_isr_exit_restore_stack(void *interrupted)
 	return (nested || next == interrupted) ? NULL : next;
 }
 
-struct {
+volatile struct {
 	void (*fn)(int, void*);
 	void *arg;
 } cpu_init[CONFIG_MP_NUM_CPUS];