Fixed ld65's precalculation of memory-area sizes.

Before this fix, BSS-type and ZP-type segments never were counted.  Now, they are counted if their memory areas are filled.  (It must be done because their places in the output file are filled.)

The fix allows us to build programs for the CBM510 and CBM610 platforms.  We won't see an "Internal error" diagnostic message about a bad file-offset.
Merge pull request #20 from greg-king5/linker

Fixed ld65's precalculation of memory-area sizes and file-offsets.