gdbserver: start turning the target ops vector into a class

gdbserver: start turning the target ops vector into a class

This is the beginning of a series of patches where the goal is to turn
the target ops vector into a class and all the target op function
pointers into methods of this class.

Currently, the target ops is a struct of function pointers.  At the
end of the series, it becomes a class with methods, and the existing
low target definitions become subclasses.  That is, we end up with the
following class hierarchy:

  process_stratum_target
  ^
  |-- linux-low
  |-- lynx-low
  |-- nto-low
  |-- win32-low

process_stratum_target either defines the default behavior for the
target ops or leaves them as pure virtual for the subclasses to
override.

The transformation is done by first introducing a helper class, called
'process_target', that is initially empty.  An instance of this class
is added to the end of the current target ops vector.  This new field
is called 'pt'.  We will gradually carry target ops to the new class,
one by one, whereas the invocation of the target op will be converted
to a method call on 'pt'.

For instance, target op 'attach' is currently invoked as

  (*the_target->attach) (args)

After moving 'attach' as a method to 'process_target', it will be
invoked as

  the_target->pt->attach (args)

In this process, the concrete target vector definitions
(e.g. linux-low, win32-low, nto-low, etc.) are turned into derived
classes of 'process_target', so that they can either inherit the
default behavior of the target ops or can override the method.

We prefer to make this transition gradually rather than in a single
giant patch, to yield bite-size patches.  The goal is that after each
patch gdbserver will still be buildable and testable.

The general rule of thumb when converting a target op to a method is
this:

(1) If the function call is protected with a NULL-check with an
obvious default behavior, simply implement that default behavior in
the base class (e.g.: supports_non_stop).

(2) If there is no NULL-check guard, the method becomes pure
virtual, and the derived targets are required to implement the method
(e.g.: attach).

(3) If there is a NULL-check but no apparent default behavior, or if
the NULL-check is utilized to populate a feature support packet,
introduce a 'supports_XYZ' method (e.g.: pid_to_exec_file).

The overall strategy is to preserve the existing behavior as much as
possible.

When we're done moving all the target ops into 'process_target', the
target op vector will contain nothing but the field 'pt'.  At that
point, the auxiliary class 'process_target' will simply meld into
'process_stratum_target' and the method calls of the form
'the_target->pt->xyz' will be turned into 'the_target->xyz'.

The "linux-low" target has been built and reg-tested on X86_64 Linux
(Ubuntu).  The "win32-low" target has been built (but not tested) via
cross-compilation to a x86_64-w64-mingw32 target.  The "lynx-low" and
"nto-low" targets were neither built nor tested.

gdbserver/ChangeLog:
2020-02-20  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* target.h (class process_target): New class definition.
	(struct process_stratum_target) <pt>: New field with type
	'process_target*'.
	* linux-low.h (class linux_process_target): Define as a derived
	class of 'process_target'.
	* linux-low.cc (linux_target_ops): Add a linux_process_target*
	as the 'pt' field.
	* lynx-low.h (class lynx_process_target): Define as a derived
	class of 'process_target'.
	* lynx-low.cc (lynx_target_ops): Add a lynx_process_target*
	as the 'pt' field.
	* nto-low.h (class nto_process_target): Define as a derived
	class of 'process_target'.
	* nto-low.cc (nto_target_ops): Add an nto_process_target*
	as the 'pt' field.
	* win32-low.h (class win32_process_target): Define as a derived
	class of 'process_target'.
	* win32-low.cc (win32_target_ops): Add a win32_process_target*
	as the 'pt' field.