Avoid keeping class loaders alive when executing ClassLoaderStatsVMOperation

The ClassLoaderStatsVMOperation is run as its own VM operation calculating and printing stats for the present class loaders. Iterating through the class loaders keeps them alive and prior to concurrent class unloading in ZGC this was no problem. With ZGC this can generate issues if the ClassLoaderStatsVMOperation occurs while doing class unloading. Classes that are dead and could be unloaded can wrongfully be kept alive just because there was a statistics operation running at the same time.