gh-110152: regrtest handles cross compilation and HOSTRUNNER (#110156)

* _add_python_opts() now handles cross compilation and HOSTRUNNER.
* display_header() now tells if Python is cross-compiled, display
  HOSTRUNNER, and get the host platform.
* Remove Tools/scripts/run_tests.py script.
* Remove "make hostrunnertest": use "make buildbottest"
  or "make test" instead.