JDK7 u40 stack size fixes

I use Archlinux, and the latest OpenJDK, and run my cassandra via @pcmanus's ccm.

When I tried the cassandra-2.0.0 branch via:

    $ ccm create dev -v git:cassandra-2.0.0

Then when I say:

    $ ccm populate -n3
    $ ccm node1 start -v

I get the following error:

    xss =  -ea -javaagent:/home/blah/.ccm/repository/1.2.2/lib/jamm-0.2.5.jar -XX:+UseThreadPriorities -XX:ThreadPriorityPolicy=42 -Xms1985M -Xmx1985M -Xmn496M -XX:+HeapDumpOnOutOfMemoryError -Xss180k
    
    The stack size specified is too small, Specify at least 228k
    Error starting node node1
    Standard error output is:
    Error: Could not create the Java Virtual Machine.
    Error: A fatal exception has occurred. Program will exit.

I tracked it down to conf/cassandra-env.sh, and changed the -Xss180k => -Xss228k and the node started.