gdbserver: turn target op 'thread_stopped' into a method

gdbserver: turn target op 'thread_stopped' into a method

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

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

	* target.h (struct process_stratum_target): Remove the target op.
	(class process_target): Add the target op.  Also add
	'supports_thread_stopped'.
	(target_thread_stopped): Update the macro.
	* target.cc (process_target::thread_stopped): Define.
	(process_target::supports_thread_stopped): Define.
	(prepare_to_access_memory): Update.

	Update the derived classes and callers below.

	* server.cc (queue_stop_reply_callback): Update.
	* linux-low.cc (linux_target_ops): Update.
	(linux_process_target::supports_thread_stopped): Define.
	(linux_thread_stopped): Turn into ...
	(linux_process_target::thread_stopped): ... 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.