* gc.c: add rb_objspace_t::flags::gc_stressfull and
  ruby_gc_stressfull macro.
  Rename objspace->gc_stress to objspace->gc_stress_mode.
  If objspace->gc_stress_mode is true (!nil and !false) then
  ruby_gc_stressfull becomes TRUE.
  ruby_gc_stressfull will speedup newobj_of() slightly.
* gc.c: initialize ruby_gc_stress(full|_mode) by gc_params.gc_stress
  even if ENABLE_VM_OBJSPACE is false.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47466 b2dd03c8-39d4-4d8f-98ff-823fe69b080e