py/nlrthumb: Fix Clang support wrt use of "return 0".

Clang defines __GNUC__ so we have to check for it specifically.

py/nlrthumb.c

@@ -76,8 +76,9 @@ __attribute__((naked)) unsigned int nlr_push(nlr_buf_t *nlr) {
 #endif
     );
 
-    #if defined(__GNUC__) && (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 8))
+    #if !defined(__clang__) && defined(__GNUC__) && (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 8))
     // Older versions of gcc give an error when naked functions don't return a value
+    // Additionally exclude Clang as it also defines __GNUC__ but doesn't need this statement
     return 0;
     #endif
 }