gdbserver: turn target op 'core_of_thread' into a method

gdbserver: turn target op 'core_of_thread' into a method

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

	Turn process_stratum_target's core_of_thread op into a method of
	process_target.

	* target.h (struct process_stratum_target): Remove the target op.
	(class process_target): Add the target op.
	(target_core_of_thread): Update the macro.
	* target.cc (process_target::core_of_thread): Define.

	Update the derived classes and callers below.

	* linux-low.cc (linux_target_ops): Update.
	(linux_process_target::core_of_thread): Define.
	* 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.