Fixed _sys_home computation and added diagnostics for Windows buildbot failures.

sysconfig.py

@@ -50,6 +50,8 @@ _sys_home = getattr(sys, '_home', None)
 if _sys_home and os.name == 'nt' and \
     _sys_home.lower().endswith(('pcbuild', 'pcbuild\\amd64')):
     _sys_home = os.path.dirname(_sys_home)
+    if _sys_home.endswith('pcbuild'):   # must be amd64
+        _sys_home = os.path.dirname(_sys_home)
 def _python_build():
     if _sys_home:
         return _is_python_source_dir(_sys_home)