Add lock to ClassLoaderDataGraph in preparation for concurrent class unloading.

New entries in the singly linked list ClassLoaderDataGraph are added via CAS. There is no performance reason to do this. It was done to be careful about safepointing while installing ClassLoaderData, but GC will find the CLD through the class loader oop.

There is various CLDG walking through the VM which now need to either take the lock or be at a safepoint.