* gc.c: Accumulate sweep time to GC time.
  Now [GC time] is [mark time] + [sweep time] + [misc].
  ([GC time] >= [mark time] + [sweep time])
* gc.c (gc_prof_sweep_slot_timer_start/stop): rename to
  gc_prof_sweep_timer_start/stop and locate at lazy_sweep().
* gc.c (elapsed_time_from): add a utility function.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41453 b2dd03c8-39d4-4d8f-98ff-823fe69b080e