GCC warns of empty format strings, perhaps because they have no
effects in printf() and there are better ways than sprintf().
However, ruby_debug_log() adds informations other than the format,
this warning is not the case.

[Misc #18891]

It's very flaky for some unknown reason. Something we have
an extra EXITED event. I suspect some other test is causing this.

* Use PRIxSIZE instead of "z"
* Fix sign-compare warning
* Suppress unused-but-set-variable warning

[Feature #18339]

After experimenting with the initial version of the API I figured there is a need
for an exit event to cleanup instrumentation data. e.g. if you record data in a
{thread_id -> data} table, you need to free associated data when a thread goes away.

follow up https://github.com/ruby/ruby/pull/6006

Ref: https://bugs.ruby-lang.org/issues/18339

Design:

- This tries to minimize the overhead when no hook is registered.
  It should only incur an extra unsynchronized boolean check.
- The hook list is protected with a read-write lock as to cause
  contention when some hooks are registered.
- The hooks MUST be thread safe, and MUST NOT call into Ruby as they
  are executed outside the GVL.
- It's simply a noop on Windows.

API:

```
rb_internal_thread_event_hook_t * rb_internal_thread_add_event_hook(rb_internal_thread_event_callback callback, rb_event_flag_t internal_event, void *user_data);
bool rb_internal_thread_remove_event_hook(rb_internal_thread_event_hook_t * hook);
```

You can subscribe to 3 events:

  - READY: called right before attempting to acquire the GVL
  - RESUMED: called right after successfully acquiring the GVL
  - SUSPENDED: called right after releasing the GVL.

The hooks MUST be threadsafe, as they are executed outside of the GVL, they also MUST NOT call any Ruby API.

This macro is no longer used ([GH-5933]).

`thread_debug()` was introduced to print debug messages
on `THREAD_DEBUG > 0` but `RUBY_DEBUG_LOG()` is more controllable.