bpo-40947: getpath.c uses PyConfig.platlibdir (GH-20807)

Followup of bpo-40854, there is one remaining usage of PLATLIBDIR
which should be replaced by config->platlibdir.

test_sys checks that sys.platlibdir attribute exists and is a string.

Update Makefile: getpath.c and sysmodule.c no longer need PLATLIBDIR
macro, PyConfig.platlibdir member is used instead.

Co-authored-by: Sandro Mani <manisandro@gmail.com>
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 2f93eaa..194128e 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -487,6 +487,7 @@
         self.assertIsInstance(sys.platform, str)
         self.assertIsInstance(sys.prefix, str)
         self.assertIsInstance(sys.base_prefix, str)
+        self.assertIsInstance(sys.platlibdir, str)
         self.assertIsInstance(sys.version, str)
         vi = sys.version_info
         self.assertIsInstance(vi[:], tuple)