Fix for over-aligned GC info
Patch by Florian Weimer

See: https://bugzilla.redhat.com/show_bug.cgi?id=1540316
Upstream discussion: https://mail.python.org/pipermail/python-dev/2018-January/152000.html

diff --git a/Include/objimpl.h b/Include/objimpl.h
index 55e83eced6..aa906144dc 100644
--- a/Include/objimpl.h
+++ b/Include/objimpl.h
@@ -248,6 +248,18 @@ PyAPI_FUNC(PyVarObject *) _PyObject_GC_Resize(PyVarObject *, Py_ssize_t);
 /* for source compatibility with 2.2 */
 #define _PyObject_GC_Del PyObject_GC_Del
 
+/* Former over-aligned definition of PyGC_Head, used to compute the
+   size of the padding for the new version below. */
+union _gc_head;
+union _gc_head_old {
+    struct {
+        union _gc_head *gc_next;
+        union _gc_head *gc_prev;
+        Py_ssize_t gc_refs;
+    } gc;
+    long double dummy;
+};
+
 /* GC information is stored BEFORE the object structure. */
 typedef union _gc_head {
     struct {
@@ -255,7 +267,8 @@ typedef union _gc_head {
         union _gc_head *gc_prev;
         Py_ssize_t gc_refs;
     } gc;
-    long double dummy;  /* force worst-case alignment */
+    double dummy;  /* force worst-case alignment */
+    char dummy_padding[sizeof(union _gc_head_old)];
 } PyGC_Head;
 
 extern PyGC_Head *_PyGC_generation0;