gdbserver: turn non-stop and async target ops into methods

gdbserver: turn non-stop and async target ops into methods

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

	Turn process_stratum_target's supports_non_stop, async, and
	start_non_stop ops into methods of process_target.

	* target.h (struct process_stratum_target): Remove the target ops.
	(class process_target): Add the target ops.
	(target_supports_non_stop): Update the macro.
	(target_async): Update the macro.
	(start_non_stop): Remove declaration.
	* target.cc (process_target::supports_non_stop): Define.
	(process_target::async): Define.
	(process_target::start_non_stop): Define.
	(start_non_stop): Remove.

	Update the derived classes and callers below.

	* server.cc (handle_qxfer_siginfo): Update.
	(handle_query): Update.
	* linux-low.cc (linux_target_ops): Update.
	(linux_supports_non_stop): Turn into ...
	(linux_process_target::supports_non_stop): ... this.
	(linux_async): Turn into ...
	(linux_process_target::async): ... this.
	(linux_start_non_stop): Turn into ...
	(linux_process_target::start_non_stop): ... this.
	* linux-low.h (class linux_process_target): Update.
	* lynx-low.cc (lynx_target_ops): Update.
	* nto-low.cc (nto_target_ops): Update.
	(nto_supports_non_stop): Remove; rely on the default behavior
	instead.
	* win32-low.cc (win32_target_ops): Update.