Avoid "-Wzero-as-null-pointer-constant" warnings on the generated code.

Reported in
<https://lists.gnu.org/archive/html/bug-gnulib/2024-11/msg00123.html>.

* src/output.cc (Output::output_lookup_function_body): To denote a null
pointer, emit a cast expression instead of plain "0".
* tests/c-parse.exp, tests/charsets.exp, tests/chill.exp,
tests/cplusplus.exp, tests/gpc.exp, tests/incomplete.exp,
tests/java.exp, tests/languages.exp, tests/modula2.exp, tests/objc.exp,
tests/permut2.exp, tests/permut3.exp, tests/permutc2.exp,
tests/test-4.exp: Update.

tests/charsets.exp

@@ -1878,6 +1878,6 @@ in_word_set (register const char *str, register size_t len)
             return &wordlist[key];
         }
     }
-  return 0;
+  return (struct charset *) 0;
 }