* include/ruby/ruby.h, gc.c: add new internal events
  RUBY_INTERNAL_EVENT_GC_ENTER and
  RUBY_INTERNAL_EVENT_GC_EXIT.
  When invoking GC process, GC_ENTER event is called.
  When exiting from GC process, GC_EXIT event is called.
  Incremental GC (incremental marking and lazy sweep) can call
  these events many times.
  For example (minor marking):
  (1) GC_ENTER
  - (2) GC_START (minor GC)
    (minor marking)
  - (3) GC_END_MARK
    (start lazy sweep)
  (4) GC_EXIT
    (ruby process)
  (5) GC_ENTER
    (lazy sweep)
  (6) GC_EXIT
    (ruby process)
    (... repeat (5), (6))
  (7) GC_ENTER
      (finish lazy sweep)
    - (8) GC_END_SWEEP
  (9) GC_EXIT
  2nd example (incremental major marking):
  (1) GC_ENTER
  - (2) GC_START (minor GC)
    (start incremental marking)
  (3) GC_EXIT
    (ruby process)
  (4) GC_ENTER
    (incremental marking)
  (5) GC_EXIT
     (ruby process)
     (... repeat (4), (5))
  (6) GC_ENTER
    (finish incremental marking)
  - (7) GC_END_MARK
    (start lazy sweep)
  (8) GC_EXIT
    (ruby process)
  (9) GC_ENTER
    (lazy sweep)
  (10) GC_EXIT
    (ruby process)
    (... repeat (9), (10))
  (11) GC_ENTER
    (finish lazy marking)
  - (12) GC_STOP_SWEEP
  (13) GC_EXIT
  Thease internal events enable to measure GC pause time completely.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47528 b2dd03c8-39d4-4d8f-98ff-823fe69b080e