Join the thread to collect zombies

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41443 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
gc.c: gc_stress is a VALUE

* gc.c (ruby_initial_gc_stress_ptr): now gc_stress is a VALUE.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41444 b2dd03c8-39d4-4d8f-98ff-823fe69b080e