singleton class of the VM object.
Before this patch, we only set mark bit for the VM object and
invoke mark function separately.
[Bug #12583]
* test/ruby/test_gc.rb: add a test.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55663 b2dd03c8-39d4-4d8f-98ff-823fe69b080e