gdb: have gdb_disassemble_info carry 'this' in its stream pointer

gdb: have gdb_disassemble_info carry 'this' in its stream pointer

The gdb_disassemble_info class is a wrapper around the libopcodes
disassemble_info struct.  The 'stream' field of disassemble_info is
passed as an argument to the fprintf_func and fprintf_styled_func
callbacks when the disassembler wants to print anything.

Previously, GDB would store a pointer to a ui_file object in the
'stream' field, then, when the disassembler wanted to print anything,
the content would be written to the ui_file object.  An example of an
fprintf_func callback, from gdb/disasm.c is:

  int
  gdb_disassembler::dis_asm_fprintf (void *stream, const char *format, ...)
  {
    /* Write output to STREAM here.  */
  }

This is fine, but has one limitation, within the print callbacks we
only have access to STREAM, we can't access any additional state
stored within the gdb_disassemble_info object.

Right now this isn't a problem, but in a future commit this will
become an issue, how we style the output being written to STREAM will
depend on the state of the gdb_disassemble_info object, and this state
might need to be updated, depending on what is being printed.

In this commit I propose changing the 'stream' field of the
disassemble_info to carry a pointer to the gdb_disassemble_info
sub-class, rather than the stream itself.

We then have the two sub-classes of gdb_disassemble_info to consider,
the gdb_non_printing_disassembler class never cared about the stream,
previously, for this class, the stream was nullptr.  With the change
to make stream be a gdb_disassemble_info pointer, no further updates
are needed for gdb_non_printing_disassembler.

The other sub-class is gdb_printing_disassembler.  In this case the
sub-class now carries around a pointer to the stream object.  The
print callbacks are updated to cast the incoming stream object back to
a gdb_printing_disassembler, and then extract the stream.

This is purely a refactoring commit.  A later commit will add
additional state to the gdb_printing_disassembler, and update the
print callbacks to access this state.

There should be no user visible changes after this commit.