Constants.java generates AccessControl Exception in unsigned applet [LUCENE-4205]

Using Lucene (i.e. writing a Document to a RAMDirectory) in an unsigned applet causes an AccessControlException because Constants.java is attempting to read a System property that is not allowed.

Caused by: java.security.AccessControlException: access denied ("java.util.PropertyPermission" "sun.arch.data.model" "read")
at java.security.AccessControlContext.checkPermission(Unknown Source)
at java.security.AccessController.checkPermission(Unknown Source)
at java.lang.SecurityManager.checkPermission(Unknown Source)
at java.lang.SecurityManager.checkPropertyAccess(Unknown Source)
at java.lang.System.getProperty(Unknown Source)
at org.apache.lucene.util.Constants.<clinit>(Constants.java:84)

on line 84: final String x = System.getProperty("sun.arch.data.model");

We have tested setting the String x to the property value for "java.vm.name" and find that works for us...

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Migrated from [LUCENE-4205](https://issues.apache.org/jira/browse/LUCENE-4205) by Stuart Rose, updated Jul 10 2012
Environment:
```
unsigned applet
```

Inspect org.apache.lucene.util.Constants.java at line 84, where the unsigned applet reads the sun.arch.data.model system property. Reproduce the failure by writing a Document to a RAMDirectory from an unsigned applet, then verify that the access-control exception no longer occurs while the architecture-related value remains usable.

Written by the indexing model from the issue text.