bpo-36722: Debug build loads libraries built in release mode (GH-12952)

In debug build, import now also looks for C extensions compiled in
release mode and for C extensions compiled in the stable ABI.
diff --git a/Python/dynload_shlib.c b/Python/dynload_shlib.c
index e5bddaa..c51f97a 100644
--- a/Python/dynload_shlib.c
+++ b/Python/dynload_shlib.c
@@ -38,9 +38,10 @@
     ".dll",
 #else  /* !__CYGWIN__ */
     "." SOABI ".so",
-#ifndef Py_DEBUG
+#ifdef ALT_SOABI
+    "." ALT_SOABI ".so",
+#endif
     ".abi" PYTHON_ABI_STRING ".so",
-#endif /* ! Py_DEBUG */
     ".so",
 #endif  /* __CYGWIN__ */
     NULL,