include/dis-asm.h patch for cgen disassemblers

[Doug Evans]

Frank Ch. Eigler writes:
 > A general hook is the "char* disassembler_options" field, which is used
 > as a catch-all.  Where this field is used (arm, i386, ppc), it's almost
 > exclusively to encode the ISA selection.

This is used to pass in disassembler options from the user too though,
right?  If so, it's definately not what you want here.

 > I'd prefer to formalize the
 > isa switch than continue to deformalize to a string option, especially
 > as isa switching is a correctness issue.  (Some of these disassembleres
 > use the string also for rendering preferences.)

As I said, I can't think of a better name.

No more objections here.