Introduced target 'atarixl'.

The target 'atarixl' is to be used for Atari XL (and better) machines.
It will disable the OS ROM and enable the Shadow RAM available on
those machine.

Note: This commit is only the inital step towards for this goal that just
replicates the target 'atari' as a starting point!

testcode/lib/div-test.c

@@ -31,10 +31,8 @@ int main(void) {
                 test(-40, -3);
         if (t)
                 printf("\nThe div() function made a wrong result!\n");
-#ifdef __ATARI__
-        /* Atari DOS 2 clears the screen after program-termination, so wait. */
+
         printf("\nTap a key, to exit. ");
         getchar();
-#endif
         return (int)t;
         }