Replace GCC's attribute optimize("Os") by the better supported and similar (cold).

Closes #2494.

Cython/Utility/ModuleSetupCode.c

@@ -671,7 +671,7 @@ static CYTHON_INLINE void * PyThread_tss_get(Py_tss_t *key) {
     #define CYTHON_SMALL_CODE
 #elif defined(__GNUC__) && (!(defined(__cplusplus)) || (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 4)))
     // At least g++ 4.4.7 can generate crashing code with this option. (GH #2235)
-    #define CYTHON_SMALL_CODE __attribute__((optimize("Os")))
+    #define CYTHON_SMALL_CODE __attribute__((cold))
 #else
     #define CYTHON_SMALL_CODE
 #endif