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.

Tools/scripts/deepfreeze.py

@@ -1,3 +1,8 @@
+"""Deep freeze
+
+The script is executed by _bootstrap_python interpreter. Shared library
+extension modules are not available.
+"""
 import argparse
 import ast
 import builtins
@@ -8,7 +13,6 @@ import re
 import sys
 import time
 import types
-import unicodedata
 from typing import Dict, FrozenSet, Tuple, TextIO
 
 import umarshal

Tools/scripts/freeze_modules.py

@@ -589,7 +589,7 @@ def regen_makefile(modules):
         ])
         deepfreezerules.append(f'{cfile}: {frozen_header} $(DEEPFREEZE_DEPS)')
         deepfreezerules.append(
-            f"\t$(PYTHON_FOR_REGEN) "
+            f"\t$(PYTHON_FOR_FREEZE) "
             f"$(srcdir)/Tools/scripts/deepfreeze.py "
             f"{frozen_header} -m {src.frozenid} -o {cfile}")
         deepfreezerules.append('')