Handle __builtin_extract_return_addr

Our architectures don't need anything special for this
builtin, just pass through the argument.

tests/tcctest.c

@@ -3027,6 +3027,8 @@ void builtin_test(void)
     printf("bce: %d\n", i);
     i = sizeof (__builtin_choose_expr (1, ll, s));
     printf("bce: %d\n", i);
+    i = sizeof (__builtin_choose_expr (0, ll, s));
+    printf("bce: %d\n", i);
 
     printf("bera: %p\n", __builtin_extract_return_addr((void*)43));
 }