[plugin_dir] use /usr/lib64 when 64-bit

Fixes issue when plugin_dir is incorrect


When using the original default setting.
`PluginDir "/usr/lib/collectd"`

```bash
$ sudo /usr/sbin/collectd -C /etc/collectd/collectd.conf -T
opendir (/usr/lib/collectd): No such file or directory
opendir (/usr/lib/collectd): No such file or directory
No read-functions are registered.
Exiting normally.
```

When using this patch
`PluginDir "/usr/lib64/collectd"`
```bash
$ sudo /usr/sbin/collectd -C /etc/collectd/collectd.conf -T
plugin_dispatch_values: No write callback has been registered. Please load at least one output plugin, if you want the collected data to be stored.
Filter subsystem: Built-in target `write': Dispatching value to all write plugins failed with status 2 (ENOENT). Most likely this means you didn't load any write plugins.
Exiting normally.
collectd: Stopping 5 read threads.
```

The no write thread warning is correct in this case of configuration. Note the correct behavior of `Stopping 5 read threads` instead of `No read-functions are registered.`