Fix fct asm label: only valid for declaration

- Fix function assembly label mechanism introduced in commit
  9b09fc376e to only accept alternative
  name for function declaration.
- merge the code with the one introduced in commit
  264a103610.
- Don't memorize token for asm label but directly the asm label.

tcc.h

@@ -219,7 +219,7 @@ typedef struct SValue {
 /* symbol management */
 typedef struct Sym {
     int v;    /* symbol token */
-    int a;    /* asm symbol token */
+    char *asm_label;    /* associated asm label */
     long r;    /* associated register */
     union {
         long c;    /* associated number */