initial support for Mega65 extra ROM

c64/kernal/iostack/setmsg.s

@@ -1,9 +1,10 @@
+#if ROM_LAYOUT_STD || (ROM_LAYOUT_M65 && SEGMENT_KERNAL_0)
 
 //
 // Official Kernal routine, described in:
 //
-// - [RG64] C64 Programmer's Reference Guide   - page 298
-// - [CM64] Compute's Mapping the Commodore 64 - page 239
+// - [RG64] C64 Programmers Reference Guide   - page 298
+// - [CM64] Computes Mapping the Commodore 64 - page 239
 //
 // CPU registers that has to be preserved (see [RG64]): .A, .X, .Y
 //
@@ -11,3 +12,6 @@
 SETMSG:
 	sta MSGFLG
 	rts
+
+
+#endif // ROM layout