[kernel] Add index to cpu_data

Because the firmware can set the APIC ids to whatever it wants, add a
sequential index to each cpu_data structure that jsix will use for its
main identifier, or for indexing into arrays, etc.

src/kernel/cpu.h

@@ -23,7 +23,9 @@ struct cpu_state
 struct cpu_data
 {
 	cpu_data *self;
-	uint64_t id;
+	uint16_t id;
+	uint16_t index;
+	uint32_t reserved;
 	uintptr_t rsp0;
 	uintptr_t rsp3;
 	TCB *tcb;