autoload: always wait on loading thread

We cannot assume autoload_provided/rb_feature_provided returning
TRUE means it is safe to proceed without waiting.  Another
thread may call rb_provide_feature before setting the constant
(via autoload_const_set).  So we must wait until autoload is
completed by another thread.

Note: this patch was tested with an explicit rb_thread_schedule
in rb_provide_feature to make the race condition more apparent
as suggested by <s.wanabe@gmail.com>:
> --- a/load.c
> +++ b/load.c
> @@ -563,6 +563,7 @@ rb_provide_feature(VALUE feature)
>      rb_str_freeze(feature);
>
>      rb_ary_push(features, rb_fstring(feature));
> +rb_thread_schedule();
>      features_index_add(feature, INT2FIX(RARRAY_LEN(features)-1));
>      reset_loaded_features_snapshot();
>  }

* variable.c (check_autoload_required): do not assume a provided
  feature means autoload is complete, always wait if autoload is
  being performed by another thread.
  [ruby-core:81105] [Bug #11384] Thanks to <s.wanabe@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58696 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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