Not restoring DS in specific for loops

[asiekierka]

Same code as #142 - but the near SwanOffset pointer has been replaced with a far one, and the bitfield access bug has been worked around by using a manually combined uint16_t write.

https://asie.pl/files/gcc-ia16-20230905-193655.i

When compiling the above file with ia16-elf-gcc -O2 -mcmodel=medium -c -o main.o gcc-ia16-20230905-193655.i, the following ASM sequence gets emitted:

  76:	89 f3                	mov    %si,%bx
  78:	d1 e3                	shl    %bx
  7a:	81 c3 00 00          	add    $0x0,%bx
			7c: R_386_16	SwanOffset
  7e:	8e 5e fe             	mov    -0x2(%bp),%ds
  81:	8b 07                	mov    (%bx),%ax

This is code to access SwanOffset[%si] and write it to %ax, which works well, except at no point is the value of %ds restored, causing issues later down the chain.

The -O0 variant of the code likewise does not restore %ds, but it seemingly avoids triggering this issue by overzealously using %ss: prefixes.