Make "benchmark" build "benchmarks", add bug note.

Makefile

@@ -283,7 +283,7 @@ BENCHMARKS=$(UPB_BENCHMARKS) \
 
 upb_benchmarks: $(UPB_BENCHMARKS)
 benchmarks: $(BENCHMARKS)
-benchmark:
+benchmark: $(BENCHMARKS)
 	@rm -f benchmarks/results
 	@rm -rf benchmarks/*.dSYM
 	@for test in benchmarks/b.* ; do ./$$test ; done

upb/atomic.h

@@ -14,6 +14,9 @@
  *
  * - a reader/writer lock (wrappers around platform-provided mutexes).
  * - an atomic refcount.
+ *
+ * TODO: This needs some revisiting/refinement, see:
+ *       http://code.google.com/p/upb/issues/detail?id=8
  */
 
 #ifndef UPB_ATOMIC_H_