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/clock.c

@@ -16,11 +16,8 @@ int main(void)
   printf("clocks per second: %d\n", CLOCKS_PER_SEC);
   printf("current clock: %ld\n", clock());
 
-#ifdef __ATARI__
-  /* Atari DOS 2.x clears the screen after program termination, so wait... */
   printf("hit <return> to exit...");
   cgetc();
-#endif
   printf("\n");
   return(0);
 }