Add marking phase to free-threaded cyclic GC

[nascheme]

Feature or enhancement

Proposal:

Like gh-126491 but implemented for the free-threaded version of the cyclic GC. This is likely a performance win for mostly the same reasons. In the free-threaded version, the marking phase avoids iterating over the mimalloc arenas as well.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-128807: Add marking phase for free-threaded cyclic GC #128808