speed up IO#close with many threads

Today, it increases IO#close performance with many threads:

  Execution time (sec)
  name            trunk   after
  vm_thread_close 4.276   3.018

  Speedup ratio: compare with the result of `trunk' (greater is better)
  name            after
  vm_thread_close 1.417

This speedup comes because rb_notify_fd_close only scans threads
inside rb_thread_io_blocking_region, not all threads in the VM.

In the future, this type data structure may allow us to notify
waiters of multiple FDs on a single thread (when using
Fibers).

* thread.c (struct waiting_fd): declare
  (rb_thread_io_blocking_region): use on-stack list waiter
  (rb_notify_fd_close): walk vm->waiting_fds instead
  (call_without_gvl): remove old field setting
  (th_init): ditto
* vm_core.h (typedef struct rb_vm_struct): add waiting_fds list
* (typedef struct rb_thread_struct): remove waiting_fd field
  (rb_vm_living_threads_init): initialize waiting_fds list

I am now kicking myself for not thinking about this 3 years ago
when I introduced ccan/list in [Feature #9632] to optimize this
same function :<

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58812 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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