[Gm2] improved debugging

[Gaius Mulley]

Hello,

I've just git pushed a rewrite of the line number debugging coordination
code in the front end.  The result is hopefully a much better experience
when single stepping Modula-2 with the (gm2/gdb) combination.  This is
turned on with the -g option from gm2.

Also introduced is another option -fm2-g which further improves the
debug line number option but this option will cause "nop" instructions
to be generated if necessary.  This option will generate accurate
debugging (line numbers) when debugging code which would not normally
generate any code.  For example consider:

   IF foo ()
   THEN
      IF bar ()
      THEN
         something
      END  (* 1 *)
   END ; (* 2 *)
   newstatement

both "END" keywords normally would not generate code.  However with the
-fm2-g option nops are generated will line number info so new
programmers can single step onto these lines.  The same is true for
REPEAT, LOOP keywords and probably others

Anyway even without using -fm2-g the new git pushed version of gm2 is
much nicer to single step and worth upgrading for imho

regards,
Gaius