Trick TASM into not creating 32-bit default segments

So that's the - admittedly rather weird - solution to the problem that has
been plaguing this project ever since the beginning of the reduction step.
Without any 32-bit dummy segments in the compiled object files, more linkers
will be able to build this project, one of them being JWlink
(http://sourceforge.net/projects/jwlink/).

Still can't rename dseg to _DATA though, as TASM stupidly refuses to accept
any ALIGN directives above a segment's alignment attribute value. TH01's
floating-point data slices already require larger alignments, and we're very
likely to have even more of those in the future.

Also, we're finally defining the Borland C++ model symbols directly in the
code, rather than in my unpublished build batch files. :)