Add a marking phase to the free-threaded GC. This is similar to what was

done in GH-126491. Since the free-threaded GC does not have generations and

is not incremental, the marking phase looks for all objects reachable from

known roots. The roots are objects known to not be garbage, like the module

dictionary for:mod:`sys`. For most programs, this marking phase should

make the GC a bit faster since typically less work is done per object.