Implement qXfer:exec-file:read in gdbserver

Implement qXfer:exec-file:read in gdbserver

This commit implements the "qXfer:exec-file:read" packet in gdbserver.

gdb/gdbserver/ChangeLog:

	* target.h (struct target_ops) <pid_to_exec_file>: New field.
	* linux-low.c (linux_target_ops): Initialize pid_to_exec_file.
	* server.c (handle_qxfer_exec_file): New function.
	(qxfer_packets): Add exec-file entry.
	(handle_query): Report qXfer:exec-file:read as supported packet.