tccgen: x86_64: fix garbage in the SValue upper bits

This was going wrong (case TOK_LAND in unary: computed labels)
-        vset(&s->type, VT_CONST | VT_SYM, 0);
-        vtop->sym = s;

This does the right thing and is shorter:

+        vpushsym(&s->type, s);


Test case was:

    int main(int argc, char **argv)
    {
        int x;
        static void *label_return = &&lbl_return;
        printf("label_return = %p\n", label_return);
        goto *label_return; //<<<<< here segfault on linux X86_64 without the memset on vset
        printf("unreachable\n");
    lbl_return:
        return 0;
    }


Also::
- Rename "void* CValue.ptr" to more usable "addr_t ptr_offset"
  and start to use it in obvious cases.

- use __attribute__ ((noreturn)) only with gnu compiler

- Revert CValue memsets ("After several days searching ...")
  commit 4bc83ac393

Doesn't mean that the vsetX/vpush thingy isn't brittle and
there still might be bugs as to differences in how the CValue
union  was set and is then interpreted later on.

However the big memset hammer was just too slow (-3% overall).

tccpp.c

@@ -974,7 +974,6 @@ static void tok_str_add2(TokenString *s, int t, CValue *cv)
 ST_FUNC void tok_str_add_tok(TokenString *s)
 {
     CValue cval;
-    memset(&cval, 0, sizeof(CValue));
 
     /* save line number info */
     if (file->line_num != s->last_line_num) {
@@ -1038,9 +1037,8 @@ static inline void TOK_GET(int *t, const int **pp, CValue *cv)
 static int macro_is_equal(const int *a, const int *b)
 {
     char buf[STRING_MAX_SIZE + 1];
-    int t;
     CValue cv;
-    memset(&cv, 0, sizeof(CValue));
+    int t;
     while (*a && *b) {
         TOK_GET(&t, &a, &cv);
         pstrcpy(buf, sizeof buf, get_tok_str(t, &cv));
@@ -1199,7 +1197,6 @@ static void tok_print(int *str)
 {
     int t;
     CValue cval;
-    memset(&cval, 0, sizeof(CValue));
 
     printf("<");
     while (1) {
@@ -2566,10 +2563,9 @@ static int *macro_arg_subst(Sym **nested_list, const int *macro_str, Sym *args)
     int last_tok, t, spc;
     const int *st;
     Sym *s;
+    CValue cval;
     TokenString str;
     CString cstr;
-    CValue cval;
-    memset(&cval, 0, sizeof(CValue));
 
     tok_str_new(&str);
     last_tok = 0;
@@ -2671,10 +2667,9 @@ static int macro_subst_tok(TokenString *tok_str,
     const int *p;
     TokenString str;
     char *cstrval;
+    CValue cval;
     CString cstr;
     char buf[32];
-    CValue cval;
-    memset(&cval, 0, sizeof(CValue));
     
     /* if symbol is a macro, prepare substitution */
     /* special macros */
@@ -2849,7 +2844,6 @@ static inline int *macro_twosharps(const int *macro_str)
     /* we search the first '##' */
     for(ptr = macro_str;;) {
         CValue cval;
-        memset(&cval, 0, sizeof(CValue));
         TOK_GET(&t, &ptr, &cval);
         if (t == TOK_TWOSHARPS)
             break;
@@ -2880,7 +2874,6 @@ static inline int *macro_twosharps(const int *macro_str)
                 t = *++ptr;
             if (t && t != TOK_TWOSHARPS) {
                 CValue cval;
-                memset(&cval, 0, sizeof(CValue));
                 TOK_GET(&t, &ptr, &cval);
                 /* We concatenate the two tokens */
                 cstr_new(&cstr);
@@ -2922,10 +2915,9 @@ static void macro_subst(TokenString *tok_str, Sym **nested_list,
     int *macro_str1;
     const int *ptr;
     int t, ret, spc;
+    CValue cval;
     struct macro_level ml;
     int force_blank;
-    CValue cval;
-    memset(&cval, 0, sizeof(CValue));
     
     /* first scan for '##' operator handling */
     ptr = macro_str;