Add an additional file = "" to memory areas that contain only bss or zp type

Add an additional file = "" to memory areas that contain only bss or zp type
segments and aren't written to the output file. Before that, the "not going to
output file" was implicit because of the bss type of the segments contained.
The new attribute makes this explicit.

git-svn-id: svn://svn.cc65.org/cc65/trunk@4853 b7a2c559-68d2-44c3-8de9-860c34a00d81