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/test/test_stat.py

@@ -4,10 +4,8 @@ import sys
 from test.support import TESTFN, import_fresh_module
 
 c_stat = import_fresh_module('stat', fresh=['_stat'])
-py_stat = import_fresh_module('stat', blocked=['_stat'])
 
 assert c_stat
-assert py_stat
 
 class TestFilemode:
     statmod = None
@@ -228,11 +226,6 @@ class TestFilemodeCStat(TestFilemode, unittest.TestCase):
     format_funcs = TestFilemode.format_funcs | {'S_ISDOOR', 'S_ISPORT',
                                                 'S_ISWHT'}
 
-
-class TestFilemodePyStat(TestFilemode, unittest.TestCase):
-    statmod = py_stat
-
-
 if __name__ == '__main__':
     unittest.main()