Call GC_on_abort (with NULL argument) on exit(1)

Call GC_on_abort (with NULL argument) on exit(1)

* include/private/gc_priv.h (EXIT): Call GC_on_abort(NULL) before
exit(1) (unless PCR) enabling the collector to enter a tight loop
(for debugging purpose) on abnormal EXIT if "GC_LOOP_ON_ABORT"
environment variable is set.
* misc.c (GC_on_abort): Allow "msg" argument to be NULL (skip
printing the message in such a case); update comment.