bpo-45950: Introduce Bootstrap Python again (#29859)

The build system now uses a :program:`_bootstrap_python` interpreter for
freezing and deepfreezing again. To speed up build process the build tools
:program:`_bootstrap_python` and :program:`_freeze_module` are no longer
build with LTO.

Cross building depends on a build Python interpreter, which must have same
version and bytecode as target host Python.