gdbserver: turn target ops 'supports_{fork,vfork,exec}_events' into methods

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

	Turn process_stratum_target's supports_fork_events,
	supports_vfork_events, and supports_exec_events 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_fork_events): Update the macro.
	(target_supports_vfork_events): Update the macro.
	(target_supports_exec_events): Update the macro.
	* target.cc (process_target::supports_fork_events): Define.
	(process_target::supports_vfork_events): Define.
	(process_target::supports_exec_events): Define.

	Update the derived classes and callers below.

	* linux-low.cc (linux_target_ops): Update.
	(linux_supports_fork_events): Turn into ...
	(linux_process_target::supports_fork_events): ... this.
	(linux_supports_vfork_events): Turn into ...
	(linux_process_target::supports_vfork_events): ... this.
	(linux_supports_exec_events): Turn into ...
	(linux_process_target::supports_exec_events): ... 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.