test_flip.rb: flip-flop shared by threads

* test/ruby/test_flip.rb (TestFlip#test_shared_thread): flip-flop
  should be separated per threads

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38747 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
vm.c: initialize root_svar

* vm.c (th_init, ruby_thread_init): initialize root_svar with Qnil,
  since lep_svar_place() expects uninitialized svar to be nil, not 0.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38748 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* load.c (load_lock): revert r38744. it should acquire new thread
  shield.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38749 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* load.c (load_lock): if thread shield is destroyed and there is no
  waiting thread, insert new thread shield into load_table.
  [Bug #7530] [ruby-core:50645]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38750 b2dd03c8-39d4-4d8f-98ff-823fe69b080e