Support multiple __label__ declarations

Support multiple __label__ declarations at the beginning of a block
as long as they're contiguous.

gcc and clang accept:
    { __label__ a,b; __label__ c;  /*...*/ }
.
Tcc would fail it. This patch makes it accept it.

The patch:
-        if (tok == TOK_LABEL) {
+        while (tok == TOK_LABEL) {

tccgen.c

@@ -6094,7 +6094,7 @@ static void block(int *bsym, int *csym, int is_expr)
         ++local_scope;
         
         /* handle local labels declarations */
-        if (tok == TOK_LABEL) {
+        while (tok == TOK_LABEL) {
             next();
             for(;;) {
                 if (tok < TOK_UIDENT)