Minimize Python startup imports (#292)

* get_exports_list should return list
* remove unintentional `CC=clang` in makefile
* avoid importing sysconfig during startup

site.py requires only a couple of functions from sysconfig, but needs to
load the entirety of sysconfig to get those functions. This commit
makes it such that sysconfig is imported only when sys.platform is darwin.

* remove redundant constants from stat module

The constants are only there in case the C implementation (ie the _stat
module) is not available. With Cosmopolitan the _stat module is always
available. The entire Lib/stat.py file can be removed if the Windows-based
constants can be moved into the Modules/_stat.c.

* minimal changes to os.py

python checks os-based assumptions at startup, some of  which can be
bypassed since this is Cosmopolitan Python.

third_party/python/Lib/os.py

@@ -38,24 +38,15 @@ def _exists(name):
     return name in globals()
 
 def _get_exports_list(module):
-    try:
-        return list(module.__all__)
-    except AttributeError:
-        return [n for n in dir(module) if n[0] != '_']
+    return list(getattr(module, "__all__", (n for n in dir(module) if n[0] != '_')))
 
 name = 'posix'
 linesep = '\n'
 from posix import *
-try:
-    from posix import _exit
-    __all__.append('_exit')
-except ImportError:
-    pass
+from posix import _exit
+__all__.append('_exit')
 import posixpath as path
-try:
-    from posix import _have_functions
-except ImportError:
-    pass
+from posix import _have_functions
 import posix
 __all__.extend(_get_exports_list(posix))
 del posix
@@ -67,7 +58,7 @@ from os.path import (curdir, pardir, sep, pathsep, defpath, extsep, altsep,
 del _names
 
 
-if _exists("_have_functions"):
+if True or _exists("_have_functions"):
     _globals = globals()
     def _add(str, fn):
         if (fn in _globals) and (str in _have_functions):