Add interrupt handlers and redo vector locations

Most of these are taken from
https://cc65.github.io/doc/customizing.html, but modified to suit this
setup.

sw/irq.c

@@ -0,0 +1,7 @@
+
+// This is defined in main.c
+void puts(const char* s);
+
+void handle_irq() {
+    puts("Interrupt Detected!\n");
+}