[01/58] 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.  This 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'.

gdbserver/ChangeLog:
2020-02-10  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* target.h (class process_target): Define.
	(struct process_stratum_target): Add a new 'pt' field, which is
	a pointer to a 'process_target'.
	* linux-low.h (class linux_process_target): Define as a derived
	class of 'process_target'.
	* linux-low.c (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.c (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.c (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.c (win32_target_ops): Add a win32_process_target*
	as the 'pt' field.
---
 gdbserver/linux-low.c |  5 +++++
 gdbserver/linux-low.h |  7 +++++++
 gdbserver/lynx-low.c  | 39 +++++++++++++++++++++++++++++++++++++++
 gdbserver/lynx-low.h  |  7 +++++++
 gdbserver/nto-low.c   | 10 ++++++++++
 gdbserver/nto-low.h   |  7 +++++++
 gdbserver/target.h    | 11 +++++++++++
 gdbserver/win32-low.c | 11 +++++++++++
 gdbserver/win32-low.h |  7 +++++++
 9 files changed, 104 insertions(+)

[01/58] gdbserver: start turning the target ops vector into a class

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