py/nlrthumb: Do not mark nlr_push as not returning anything.

By adding __builtin_unreachable() at the end of nlr_push, we're
essentially telling the compiler that this function will never return.
When GCC LTO is in use, this means that any time nlr_push() is called
(which is often), the compiler thinks this function will never return
and thus eliminates all code following the call.

Note: I've added a 'return 0' for older GCC versions like 4.6 which
complain about not returning anything (which doesn't make sense in a
naked function). Newer GCC versions (tested 4.8, 5.4 and some others)
don't complain about this.

py/nlrthumb.c

@@ -76,9 +76,9 @@ __attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) {
 #endif
     );
 
-    #if defined(__GNUC__)
+    #if defined(__GNUC__) && (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 8))
     // Older versions of gcc give an error when naked functions don't return a value
-    __builtin_unreachable();
+    return 0;
     #endif
 }