[gdb, hurd] Avoid using 'PATH_MAX' in 'gdb/remote.c'

[gdb, hurd] Avoid using 'PATH_MAX' in 'gdb/remote.c'

..., which is not defined in GNU/Hurd systems, and so commit
94585166dfea8232c248044f9f4b1c217dc4ac2e "Extended-remote follow-exec" caused:

    [...]/gdb/remote.c: In member function 'void remote_target::remote_parse_stop_reply(const char*, stop_reply*)':
    [...]/gdb/remote.c:7343:22: error: 'PATH_MAX' was not declared in this scope
            char pathname[PATH_MAX];
                          ^~~~~~~~

	gdb/
	* remote.c (remote_target::remote_parse_stop_reply): Avoid using
	'PATH_MAX'.