initial support for Mega65 extra ROM

c64/kernal/interrupts/fffe.vector_irq.s

@@ -1,5 +1,10 @@
+#if ROM_LAYOUT_STD || (ROM_LAYOUT_M65 && SEGMENT_KERNAL_0)
+
 // $FFFE - CPU IRQ Vector.
 // This is non-controvertial, as it is a 6502 CPU requirement, and we just have to point
 // it to whatever IRQ routine we make.
 
 	.word hw_entry_irq
+
+
+#endif // ROM layout