tccgen: asm_label cleanup

- avoid memory allocation by using its (int) token number
- avoid additional function parameter by using Attribute

Also: fix some strange looking error messages

tcc.h

@@ -434,12 +434,13 @@ typedef struct AttributeDef {
     struct Attribute a;
     struct Section *section;
     int alias_target;    /* token */
+    int asm_label;    /* associated asm label */
 } AttributeDef;
 
 /* symbol management */
 typedef struct Sym {
     int v;    /* symbol token */
-    char *asm_label;    /* associated asm label */
+    int asm_label;    /* associated asm label */
     union {
         long r;    /* associated register */
         struct Attribute a;