* gc.c (gc_writebarrier_generational): add an alternative write

  barrier (WB) implementation.
  When finding reference from [Old obj] to [New obj] by WB, current
  implementation marks [Old obj] as remembered old objects and marks
  chilldren of [Old obj] at the beggining of marking.
  Added (but disabled) code changes current behaviour. This fix promote
  [New obj] to old and marks as a remembered old object. We can assume
  "new objects referred from old objects are maybe long-lived old
  objects".
  Disadvantage of added algorithm is we may promote unwilling
  short-lived objects. For example, consider many new objects push and
  pop to an old stack object. All of new objects (short-lived objects)
  promote to old objects unexpectedly.
  To compare these behaviour, I add this new code (but disabled it).



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50006 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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