gdbserver: turn prepare_to_access_memory & done_accessing_memory into methods

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

	Turn process_stratum_target's prepare_to_access_memory and
	done_accessing_memory ops into methods of process_target.

	* target.h (struct process_stratum_target): Remove the target ops.
	(class process_target): Add the target ops.
	* target.cc (process_target::prepare_to_access_memory): Define.
	(process_target::done_accessing_memory): Define.
	(prepare_to_access_memory): Update.
	(done_accessing_memory): Update.

	Update the derived classes and callers below.

	* linux-low.cc (linux_target_ops): Update.
	(linux_prepare_to_access_memory): Turn into ...
	(linux_process_target::prepare_to_access_memory): ... this.
	(linux_done_accessing_memory): Turn into ...
	(linux_process_target::done_accessing_memory): ... this.
	* linux-low.h (class linux_process_target): Update.
	* lynx-low.cc (lynx_target_ops): Update.
	* nto-low.cc (nto_target_ops): Update.
	* win32-low.cc (win32_target_ops): Update.