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/strnicmp-test.c

@@ -1,10 +1,7 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
-#ifdef __ATARI__
-#include <atari.h>
 #include <conio.h>
-#endif
 
 static int do_test(const char *s1, const char *s2, size_t n)
 {
@@ -70,11 +67,6 @@ int main(void)
     else
         printf("OK (%d)\n", ret);
 
-#ifdef __ATARI__
-    if (_dos_type != 1) {
-        cgetc();
-    }
-#endif
-
+    cgetc();
     return 0;
 }