[ruby/mmtk] Add VM barrier in rb_gc_impl_before_fork

We need the VM barrier in rb_gc_impl_before_fork to stop the other Ractors
because otherwise they could be allocating objects in the fast path which
could be calling mmtk_add_obj_free_candidate. Since mmtk_add_obj_free_candidate
acquires a lock on obj_free_candidates in weak_proc.rs, this lock may not
be released in the child process after the Ractor dies.

For example, the following script demonstrates the issue:

    puts "Hello #{Process.pid}"

    100.times do |i|
      puts "i = #{i}"
      Ractor.new(i) do |j|
        puts "Ractor #{j} hello"
        1000.times do |i|
          s = "#{j}-#{i}"
        end
        Ractor.receive
        puts "Ractor #{j} goodbye"
      end
      pid = fork { }
      puts "Child pid is #{pid}"
      _, status = Process.waitpid2 pid
      puts status.success?
    end

    puts "Goodbye"

In the child process, we can see that it is stuck trying to acquire the
lock on obj_free_candidates:

    #5  0x00007192bfb53f10 in mmtk_ruby::weak_proc::WeakProcessor::get_all_obj_free_candidates (self=0x7192c0657498 <mmtk_ruby::BINDING+72>) at src/weak_proc.rs:52
    #6  0x00007192bfa634c3 in mmtk_ruby::api::mmtk_get_all_obj_free_candidates () at src/api.rs:295
    #7  0x00007192bfa61d50 in rb_gc_impl_shutdown_call_finalizer (objspace_ptr=0x578c17abfc50) at gc/mmtk/mmtk.c:1032
    #8  0x0000578c1601e48e in rb_ec_finalize (ec=0x578c17ac06d0) at eval.c:166
    #9  rb_ec_cleanup (ec=<optimized out>, ex=<optimized out>) at eval.c:257
    #10 0x0000578c1601ebf6 in ruby_cleanup (ex=<optimized out>) at eval.c:180
    #11 ruby_stop (ex=<optimized out>) at eval.c:292
    #12 0x0000578c16127124 in rb_f_fork (obj=<optimized out>) at process.c:4291
    #13 rb_f_fork (obj=<optimized out>) at process.c:4281

https://github.com/ruby/mmtk/commit/eb4b229858

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