[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.cpp

@@ -57,9 +57,6 @@ cpu_init(cpu_data *cpu, bool bsp)
 		cpu_early_init(cpu);
 	}
 
-	lapic &apic = device_manager::get().get_lapic();
-	cpu->id = apic.get_id();
-
 	// Set up the syscall MSRs
 	syscall_enable();