* gc.c (gc_mark_roots): should mark the VM object itself to mark
	  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/branches/ruby_2_2@55921 b2dd03c8-39d4-4d8f-98ff-823fe69b080e