gdbserver: constify the 'pid_to_exec_file' target op

gdbserver/ChangeLog:
2021-04-12  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* target.h (class process_stratum_target) <pid_to_exec_file>:
	Constify the return type.  Update the definition/references below.
	* target.cc (process_stratum_target::pid_to_exec_file)
	* linux-low.h (class linux_process_target) <pid_to_exec_file>
	* linux-low.cc (linux_process_target::pid_to_exec_file)
	* netbsd-low.h (class netbsd_process_target) <pid_to_exec_file>
	* netbsd-low.cc (netbsd_process_target::pid_to_exec_file)
	* server.cc (handle_qxfer_exec_file)