gh-117139: Garbage collector support for deferred refcounting (#122956)

gh-117139: Garbage collector support for deferred refcounting (#122956)

The free-threaded GC now visits interpreter stacks to keep objects
that use deferred reference counting alive.

Interpreter frames are zero initialized in the free-threaded GC so
that the GC doesn't see garbage data. This is a temporary measure
until stack spilling around escaping calls is implemented.

Co-authored-by: Ken Jin <kenjin@python.org>