gh-128807: Add marking phase for free-threaded cyclic GC

[nascheme]

This is conceptually similar to the phase that was added to the non-free-threaded GC. Start with a set of known root objects, like sysdict and mark all objects reachable from those (revealed by the tp_traverse method) as "alive". We know anything marked alive cannot be garbage and can be excluded from the regular cyclic GC process. For most programs, this saves a moderate amount of computation since the marking pass is relatively cheaper per object.

If gc.freeze() is used, it's unlikely that this marking phase will be a win since it's expected that the majority of objects will be frozen. Disable the marking phase if freeze is used.

See gh-126491 for the non-free-threaded version of this technique.

pyperformance results vs merge base. I suspect the slowdown on some benchmarks is not real. For example, regex_v8 should not be slower.

Here are the pyperformance results for a bare-metal AMD Ryzen machine. It does not show a slowdown on regex_v8, for example.

To better show the expected improvement, I ran a "sphinx build" benchmark, like in gh-124567. Results are:

	old	new
GC collections	38	38
long-lived objects	586,986	585,730
total run time	3.24 s	3.03 s
mark phase time	0.00 s	0.16 s
total gc time	0.72 s	0.57 s

• Issue: Add marking phase to free-threaded cyclic GC #128807

[ghost]

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[colesbury]

Looks great! A few comments below

[colesbury]

LGTM