Perform build and magnum tuning

Building o//third_party/python now takes 5 seconds on my PC

This change works towards modifying Python to use runtime dispatching
when appropriate. For example, when loading the magnums in the socket
module, it's a good idea to check if the magnum is zero, because that
means the local system platform doesn't support it.

third_party/python/Include/codecs.h

@@ -1,8 +1,7 @@
 #ifndef Py_CODECREGISTRY_H
 #define Py_CODECREGISTRY_H
-#ifdef __cplusplus
-extern "C" {
-#endif
+COSMOPOLITAN_C_START_
+/* clang-format off */
 
 /* ------------------------------------------------------------------------
 
@@ -234,7 +233,5 @@ PyAPI_FUNC(PyObject *) PyCodec_NameReplaceErrors(PyObject *exc);
 PyAPI_DATA(const char *) Py_hexdigits;
 #endif
 
-#ifdef __cplusplus
-}
-#endif
+COSMOPOLITAN_C_END_
 #endif /* !Py_CODECREGISTRY_H */