Switch to Claspath's java.lang.Thread

Rather than doing this as a separate task it was lumped in with the work on the RVM-91 branch. I will extract out and commit in reasonable sized chunks to main trunk.

Hi Peter, whilst doing this change looks quite simple there are a number of subtle fixes in the RVM-91 branch that enable the Classpath java.lang.Thread to work there. I'll list the ones I can remember:

• Thread.suspend and Thread.resume are synchronized meaning that Thread.suspend must unlock the lock held on itself prior to suspending
  • this leads to problems unwinding the stack as a synchronized method should hold a lock, whereas now we need to explicitly release it
• the creation of VM threads needs to be modified so that we can use Classpath's rather than our own hooks
  • one particular point of pain is that a java.lang.Thread is created prior to its VM_Thread. The VM_Thread is only created and started by a start happening.
  • another problem is with setting up the java.lang.Thread for the boot thread
    Whilst lumping the work together is far from ideal I wanted to have a good stab at debugging all the changes in the RVM-91 branch together. I've already spent twice the time I'd hope to getting this code into shape.
    Thank you for putting some small changes from the RVM-91 branch into the trunk. I'm not sure if doing the rest of these changes independent of RVM-91 is a good idea. In particular, it seems likely fixes to get this to work will conflict with the hopefully superior way of doing things in the RVM-91 branch.

This has been completed as part of RVM-64 and when that merges back into trunk this will be done.