* gc.c, gc.h (rb_objspace_marked_object_p): added.
  This function *ONLY* works just after marking phase,
  before any sweeping.
  This function is highly depending current GC implementation
  and can be removed future version.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44886 b2dd03c8-39d4-4d8f-98ff-823fe69b080e