pythongh-129201: Use prefetch in GC mark alive phase. (pythongh-129203)

For the free-threaded version of the cyclic GC, restructure the "mark alive" phase to use software prefetch instructions.  This gives a speedup in most cases when the number of objects is large enough.  The prefetching is enabled conditionally based on the number of long-lived objects the GC finds.

Author: nascheme

Misc/NEWS.d/next/Core_and_Builtins/2025-01-22-14-22-34.gh-issue-129201.wiZzEb.rst

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+The free-threaded version of the cyclic garbage collector has been optimized to
+conditionally use CPU prefetch instructions during the collection.  This can
+reduce collection times by making it more likely that data is in the CPU cache
+when it is needed.  The prefetch instructions are enabled if the number of
+long-lived objects (objects surviving a full collection) exceeds a threshold.