More robust memory limits

[bobbimanners]

At present I am setting memory limits manually. I review the cc65 .map file and find the upper limit of the memory used by cc65 and then set my HEAP1 and HEAP2 limits accordingly. Oliver Schmidt suggested a better way using malloc() and _heapmaxavail. See cc65 issue #655 for more details. This applies to Apple II only, as I am not linking in malloc() on VIC20/C64.