gdbserver: turn target op 'hostio_last_error' into a method

gdbserver: turn target op 'hostio_last_error' into a method

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

	Turn process_stratum_target's hostio_last_error 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.cc: Add "hostio.h" to includes.
	(process_target::hostio_last_error): Define.

	Update the derived classes and callers below.

	* hostio.cc (hostio_error): Update.
	* linux-low.cc: Remove "hostio.h" from includes.
	(linux_target_ops): Update.
	* lynx-low.cc (lynx_target_ops): Update.
	* nto-low.cc (nto_target_ops): Update.
	* win32-low.h (class win32_process_target): Update.
	* win32-low.cc (win32_target_ops): Update.
	(wince_hostio_last_error): Turn into ...
	(win32_process_target::hostio_last_error): ... this.