gdb: add new base class to gdb_disassembler

gdb: add new base class to gdb_disassembler

The motivation for this change is an upcoming Python disassembler API
that I would like to add.  As part of that change I need to create a
new disassembler like class that contains a disassemble_info and a
gdbarch.  The management of these two objects is identical to how we
manage these objects within gdb_disassembler, so it might be tempting
for my new class to inherit from gdb_disassembler.

The problem however, is that gdb_disassembler has a tight connection
between its constructor, and its print_insn method.  In the
constructor the ui_file* that is passed in is replaced with a member
variable string_file*, and then in print_insn, the contents of the
member variable string_file are printed to the original ui_file*.

What this means is that the gdb_disassembler class has a tight
coupling between its constructor and print_insn; the class just isn't
intended to be used in a situation where print_insn is not going to be
called, which is how my (upcoming) sub-class would need to operate.

My solution then, is to separate out the management of the
disassemble_info and gdbarch into a new gdb_disassemble_info class,
and make this class a parent of gdb_disassembler.

In arm-tdep.c and mips-tdep.c, where we used to cast the
disassemble_info->application_data to a gdb_disassembler, we can now
cast to a gdb_disassemble_info as we only need to access the gdbarch
information.

Now, my new Python disassembler sub-class will still want to print
things to an output stream, and so we will want access to the
dis_asm_fprintf functionality for printing.

However, rather than move this printing code into the
gdb_disassemble_info base class, I have added yet another level of
hierarchy, a gdb_printing_disassembler, thus the class structure is
now:

  struct gdb_disassemble_info {};
  struct gdb_printing_disassembler : public gdb_disassemble_info {};
  struct gdb_disassembler : public gdb_printing_disassembler {};

In a later commit my new Python disassembler will inherit from
gdb_printing_disassembler.

The reason for adding the additional layer to the class hierarchy is
that in yet another commit I intend to rewrite the function
gdb_buffered_insn_length, and to do this I will be creating yet more
disassembler like classes, however, these will not print anything,
thus I will add a gdb_non_printing_disassembler class that also
inherits from gdb_disassemble_info.  Knowing that that change is
coming, I've gone with the above class hierarchy now.

There should be no user visible changes after this commit.