abs_paths() is the another most slow parts of site module.
We can speedup it if we have C implementation of os.path.abspath() or abs_paths().

But before trying it, is abs_paths() really needed for now?
It only rewrite `__file__` and `__cached__` of modules imported before site is imported.
What is difference between modules loaded befere / after site module?

Here is profile output. sysconfig dependency is removed by #29585 patch, and pstat is modified
to show time in ms instead of sec.

   Ordered by: cumulative time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
      3/1    0.004    0.001    2.525    2.525 {built-in method builtins.exec}
        1    0.003    0.003    2.524    2.524 x.py:1(<module>)
        1    0.010    0.010    1.806    1.806 site.py:555(main)
      4/3    0.022    0.005    1.179    0.393 <frozen importlib._bootstrap>:958(_find_and_load)
      4/3    0.017    0.004    1.110    0.370 <frozen importlib._bootstrap>:931(_find_and_load_unlocked)
        1    0.195    0.195    0.928    0.928 site.py:99(abs_paths)
      108    0.098    0.001    0.776    0.007 posixpath.py:367(abspath)
        4    0.035    0.009    0.647    0.162 <frozen importlib._bootstrap>:861(_find_spec)
        4    0.005    0.001    0.589    0.147 <frozen importlib._bootstrap_external>:1150(find_spec)
        4    0.043    0.011    0.584    0.146 <frozen importlib._bootstrap_external>:1118(_get_spec)
      2/1    0.012    0.006    0.557    0.557 <frozen importlib._bootstrap>:641(_load_unlocked)
      2/1    0.006    0.003    0.511    0.511 <frozen importlib._bootstrap_external>:673(exec_module)
      108    0.461    0.004    0.493    0.005 posixpath.py:329(normpath)
       16    0.150    0.009    0.453    0.028 <frozen importlib._bootstrap_external>:1234(find_spec)