Add two load chunks:
 - First chunk checks system (if it's XL and memory is ok).
 - Second chunk contains the contents for the shadow memory beneath the
   ROM and copies the contents to their destination address.
   It also moves the screen memory to low memory in front of the program.
Some small first changes for the 'atarixl' target.
"save area" is used to save modified system variables and ports. It's
shared between the second and main chunk.
Code & data for "system check" load chunk in the atarixl target.
Code & data for "shadow RAM preparation" load chunk in the atarixl target.
Intermediate check-in, unfinished...