Fix cpu.inc guard to check for actual CPU symbol

- Change guard from checking __CPU_INC_INCLUDED__ to CPU_ISET_NONE
- This properly detects if cpu.mac has already been included
- Fixes the 'Symbol already defined' errors for all apple2 targets

The issue was that .ifndef __CPU_INC_INCLUDED__ didn't prevent the
.macpack cpu from being executed multiple times, since the CPU symbols
are defined with = (assignment) not .define. Checking for CPU_ISET_NONE
(which is defined by cpu.mac) properly guards against multiple inclusion.

Author: dillera

common/inc/cpu.inc

@@ -1,6 +1,6 @@
 ; Guard to prevent multiple inclusion of cpu macros
-.ifndef __CPU_INC_INCLUDED__
-.define __CPU_INC_INCLUDED__
+; Check if CPU_ISET_NONE is already defined (from cpu.mac)
+.ifndef CPU_ISET_NONE
 
 .macpack cpu