lock: Improve performance of DEBUG_THREADS.

Add a volatile flag to lock tracking structures so we only need to use
the global lock when first initializing tracking.

Additionally add support for DEBUG_THREADS_LOOSE_ABI.  This is used by
astobj2.c to eliminate storage for tracking fields when DEBUG_THREADS is
not defined.

Change-Id: Iabd650908901843e9fff47ef1c539f0e1b8cb13b