bpo-44860: Make sysconfig posix_user not depend on platlibdir (GH-27655) (GH-28197)

(cherry picked from commit a5c6bcf24479934fe9c5b859dd1cf72685a0003a)

Co-authored-by: Tzu-ping Chung <uranusjr@gmail.com>
diff --git a/Lib/sysconfig.py b/Lib/sysconfig.py
index d700880..95b48f6 100644
--- a/Lib/sysconfig.py
+++ b/Lib/sysconfig.py
@@ -101,7 +101,7 @@ def joinuser(*args):
             'stdlib': '{userbase}/{platlibdir}/python{py_version_short}',
             'platstdlib': '{userbase}/{platlibdir}/python{py_version_short}',
             'purelib': '{userbase}/lib/python{py_version_short}/site-packages',
-            'platlib': '{userbase}/{platlibdir}/python{py_version_short}/site-packages',
+            'platlib': '{userbase}/lib/python{py_version_short}/site-packages',
             'include': '{userbase}/include/python{py_version_short}',
             'scripts': '{userbase}/bin',
             'data': '{userbase}',