[gcc-ia16] Put literal strings and constant structures in ROM memory by default

[asiekierka]

Right now, constant structures have to be explicitly annotated with __far to be placed in ROM memory. This is less than ideal:

• Existing code most likely doesn't use the __far annotation, requiring adaptation;
• Literal strings cannot be placed in ROM memory at all.

There are many options to improve this:

• Patch gcc-ia16 to place literal strings in .fartext and as __far-annotated variables;
• Patch gcc-ia16 to place read-only data in .farrodata and as __far-annotated unless __near is present;
• Patch gcc-ia16 to support placing variables in the code segment; this would allow using CS-relative pointers for a slight performance benefit compared to __far annotations for literal strings.
• Add a large memory model to gcc-ia16, as the times when we want a near pointer are notably less numerious than times when we want a far pointer.

This is a rather difficult task, unfortunately.