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.

Programs/_bootstrap_python.c

@@ -0,0 +1,105 @@
+
+/* Frozen modules bootstrap
+ *
+ * Limited and restricted Python interpreter to run
+ * "Tools/scripts/deepfreeze.py" on systems with no or older Python
+ * interpreter.
+ */
+
+#include "Python.h"
+#include "pycore_import.h"
+
+/* Includes for frozen modules: */
+#include "Python/frozen_modules/importlib._bootstrap.h"
+#include "Python/frozen_modules/importlib._bootstrap_external.h"
+/* End includes */
+
+/* Note that a negative size indicates a package. */
+
+static const struct _frozen bootstrap_modules[] = {
+    {"_frozen_importlib", _Py_M__importlib__bootstrap, (int)sizeof(_Py_M__importlib__bootstrap)},
+    {"_frozen_importlib_external", _Py_M__importlib__bootstrap_external, (int)sizeof(_Py_M__importlib__bootstrap_external)},
+    {0, 0, 0} /* bootstrap sentinel */
+};
+static const struct _frozen stdlib_modules[] = {
+    {0, 0, 0} /* stdlib sentinel */
+};
+static const struct _frozen test_modules[] = {
+    {0, 0, 0} /* test sentinel */
+};
+const struct _frozen *_PyImport_FrozenBootstrap = bootstrap_modules;
+const struct _frozen *_PyImport_FrozenStdlib = stdlib_modules;
+const struct _frozen *_PyImport_FrozenTest = test_modules;
+
+static const struct _module_alias aliases[] = {
+    {"_frozen_importlib", "importlib._bootstrap"},
+    {"_frozen_importlib_external", "importlib._bootstrap_external"},
+    {0, 0} /* aliases sentinel */
+};
+const struct _module_alias *_PyImport_FrozenAliases = aliases;
+
+/* Embedding apps may change this pointer to point to their favorite
+   collection of frozen modules: */
+
+const struct _frozen *PyImport_FrozenModules = NULL;
+
+int
+#ifdef MS_WINDOWS
+wmain(int argc, wchar_t **argv)
+#else
+main(int argc, char **argv)
+#endif
+{
+    PyStatus status;
+
+    PyConfig config;
+    PyConfig_InitIsolatedConfig(&config);
+    // don't warn, pybuilddir.txt does not exist yet
+    config.pathconfig_warnings = 0;
+    // parse arguments
+    config.parse_argv = 1;
+    // add current script dir to sys.path
+    config.isolated = 0;
+
+#ifdef MS_WINDOWS
+    status = PyConfig_SetArgv(&config, argc, argv);
+#else
+    status = PyConfig_SetBytesArgv(&config, argc, argv);
+#endif
+    if (PyStatus_Exception(status)) {
+        goto error;
+    }
+
+    status = PyConfig_Read(&config);
+    if (config.run_filename == NULL) {
+        status = PyStatus_Error("Run filename expected");
+        goto error;
+    }
+
+#define CLEAR(ATTR) \
+    do { \
+        PyMem_RawFree(ATTR); \
+        ATTR = NULL; \
+    } while (0)
+
+    // isolate from system Python
+    CLEAR(config.base_prefix);
+    CLEAR(config.prefix);
+    CLEAR(config.base_exec_prefix);
+    CLEAR(config.exec_prefix);
+
+    status = Py_InitializeFromConfig(&config);
+    if (PyStatus_Exception(status)) {
+        goto error;
+    }
+    PyConfig_Clear(&config);
+
+    return Py_RunMain();
+
+error:
+    PyConfig_Clear(&config);
+    if (PyStatus_IsExit(status)) {
+        return status.exitcode;
+    }
+    Py_ExitStatusException(status);
+}