T_DATA with a NULL pointer are not marked. Let's wrap 1 instead to
ensure that our mark function is actually run.

Followup: https://github.com/ruby/ruby/pull/9029

[Bug #20019]

Some events still weren't triggered from the right place.

The test suite was also improved a bit more.

This entirely changes how it is tested. Rather than to use counters
we now record the timeline of events with associated threads which
makes it much easier to assert that certains events are only preceded
by a specific event, and makes it much easier to debug unexpected
timelines.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>
Co-Authored-By: JP Camara <jp@jpcamara.com>
Co-Authored-By: John Hawthorn <john@hawthorn.email>

Context: https://github.com/ivoanjo/gvl-tracing/pull/4

Some hooks may want to collect data on a per thread basis.
Right now the only way to identify the concerned thread is to
use `rb_nativethread_self()` or similar, but even then because
of the thread cache or MaNy, two distinct Ruby threads may report
the same native thread id.

By passing `thread->self`, hooks can use it as a key to store
the metadata.

NB: Most hooks are executed outside the GVL, so such data collection
need to use a thread-safe data-structure, and shouldn't use the
reference in other ways from inside the hook.

They must also either pin that value or handle compaction.

Checking by `__STDC_VERSION__` is unreliable because old gcc 4.8
supports `-std=gnu11` option but does not implement `_Thread_local`.
Check the implementation directly instead.

Split `PACKED_STRUCT` and `PACKED_STRUCT_UNALIGNED` macros into the
macros bellow:
* `RBIMPL_ATTR_PACKED_STRUCT_BEGIN`
* `RBIMPL_ATTR_PACKED_STRUCT_END`
* `RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_BEGIN`
* `RBIMPL_ATTR_PACKED_STRUCT_UNALIGNED_END`