gdbserver: turn target ops 'read_pc' and 'write_pc' into methods

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

	Turn process_stratum_target's read_pc and write_pc ops into
	methods of process_target.

	* target.h (struct process_stratum_target): Remove the target ops.
	(class process_target): Add the target ops.
	* target.cc (process_target::read_pc): Define.
	(process_target::write_pc): Define.

	Update the derived classes and callers below.

	* regcache.cc (regcache_read_pc): Update.
	(regcache_write_pc): Update.
	* linux-low.cc (linux_target_ops): Update.
	(linux_read_pc): Turn into ...
	(linux_process_target::read_pc): ... this.
	(linux_write_pc): Turn into ...
	(linux_process_target::write_pc): ... 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.