x86: Better disassembly of segment-prefixes.

x86: Better disassembly of segment-prefixes.

Thanks to Alastair Bridgewater who originally provided these changes
(and the headline above). I adapted his work to fit into the prefix
instruction infrastructure now available. Of his original comments
the following three still apply:

  * Establish a SEG prefix for segment overrides similar
to the X66 data-width prefix.

  * Have the SEG prefilter set an instruction property
for the specific segment being used.

  * Alter PRINT-MEM-ACCESS to output a suitable prefix
for memory addresses when the appropriate instruction
property has been set.

I have abstracted out the segment prefix printing into the new function
MAYBE-PRINT-SEGMENT-OVERRIDE, called from PRINT-MEM-ACCESS, not to make
the latter more lengthy.

Here is an example to show the difference in disassembler output:

Old:

;      0E6:       64               FS-SEGMENT-PREFIX
;      0E7:       8910             MOV [EAX], EDX
;      0E9:       64               FS-SEGMENT-PREFIX
;      0EA:       8B0528000000     MOV EAX, [#x28]

New:

;      0E6:       648910           MOV FS:[EAX], EDX
;      0E9:       648B0528000000   MOV EAX, FS:[#x28]