[~kbarrett] suggested instead of encoding the SATBMarkQueue::_active flag into _index in JDK-8227002, use a special _buf == NULL value to do that.

This simplifies the generated pre-barrier code, decreases the size of the data in the GC thread local buffer and obsoletes the need for SATBMarkQueue::_active.

This seems easier to do and require much less compiler wrangling than JDK-8227002.

An empty buffer (as opposed to no buffer) could still be encoded as 0-sized non-NULL _buf value if needed.

[Updated for JDK-8258255, which moved the active flag from PtrQueue to SATBMarkQueue.]