gh-84461: Fix Emscripten umask and permission issues (GH-94002)

- Emscripten's default umask is too strict, see
  https://github.com/emscripten-core/emscripten/issues/17269
- getuid/getgid and geteuid/getegid are stubs that always return 0
  (root). Disable effective uid/gid syscalls and fix tests that use
  chmod() current user.
- Cannot drop X bit from directory.

Lib/test/test_pydoc.py

@@ -934,6 +934,7 @@ class PydocImportTest(PydocBaseTest):
         self.assertEqual(err.getvalue(), '')
 
     @os_helper.skip_unless_working_chmod
+    @unittest.skipIf(is_emscripten, "cannot remove x bit")
     def test_apropos_empty_doc(self):
         pkgdir = os.path.join(TESTFN, 'walkpkg')
         os.mkdir(pkgdir)