Fix id2ref table build when GC in progress

Previously, if GC was in progress when we're initially building the
id2ref table, it could see the empty table and then crash when trying to
remove ids from it. This commit fixes the bug by only publishing the
table after GC is done.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

test/ruby/test_objectspace.rb

@@ -284,6 +284,21 @@ End
     end;
   end
 
+  def test_id2ref_table_build
+    assert_separately([], <<-End)
+      10.times do
+        Object.new.object_id
+      end
+
+      GC.start(immediate_mark: false)
+
+      obj = Object.new
+      EnvUtil.suppress_warning do
+        assert_equal obj, ObjectSpace._id2ref(obj.object_id)
+      end
+    End
+  end
+
   def test_each_object_singleton_class
     assert_separately([], <<-End)
       class C