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merge revision(s) 57410,57619,57621,57631,57634: [Backport #13150]

    Prevent GC by volatile [Bug #13150]

    test/ruby/test_marshal.rb test_context_switch (load) and test_gc (dump)
    are failed on FreeBSD 10.3 and gcc7 (FreeBSD Ports Collection) 7.0.0
    20170115 (experimental); RB_GC_GUARD looks not worked well.
    ruby.h: RB_GC_GUARD stronger than gcc7

    * include/ruby/ruby.h (RB_GC_GUARD): prevent guarded pointer from
      optimization by using as an input to inline asm.
    ruby.h: remove comment

    * include/ruby/ruby.h (RB_GC_GUARD): remove comment unsupported by
      Solaris AS.
    marshal.c: use hidden objects to allow recycling

    Hidden objects (klass == 0) are not visible to Ruby code invoked
    from other threads or signal handlers, so they can never be
    accessed from other contexts.  This makes it safe to call
    rb_gc_force_recycle on the object slot after releasing malloc
    memory.

    * marshal.c (rb_marshal_dump_limited): hide dump_arg and recycle when done
      (rb_marshal_load_with_proc): hide load_arg and recycle when done
      [ruby-core:79518]
    marshal.c: revert r57631 partially

    * marshal.c (rb_marshal_dump_limited): do not free dump_arg, which
      may be dereferenced in check_dump_arg due to continuation, and
      get rid of dangling pointers.

    * marshal.c (rb_marshal_load_with_proc): ditto for load_arg.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57954 b2dd03c8-39d4-4d8f-98ff-823fe69b080e