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

gh-129201: Use prefetch in GC mark alive phase. (gh-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.