* gc.c, internal.h (rb_gc_writebarrier_remember_promoted): add a new
  function to remember an specified object. This api is only
  experimental (strongly depend on WB/rgengc strategy).



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42062 b2dd03c8-39d4-4d8f-98ff-823fe69b080e