gh-140358: Bring back elapsed time and unreachable count to gc debug output

[pgdr]

Fixes #140358

This code includes pycore_time.h in gc.c and times the run of _PyGC_Collect. If _PyGC_DEBUG_STATS, then we print the total time used:

E.g.

gc: done, 0 unreachable, 0 uncollectable, 0.0741s elapsed

See also discussion at elapsed time debugging in gc.c at discuss.

• Issue: Bring back elapsed time debugging in gc.c #140358