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).

tccgen.c

@@ -329,7 +329,6 @@ static void vsetc(CType *type, int r, CValue *vc)
 void vpush(CType *type)
 {
     CValue cval;
-    memset(&cval, 0, sizeof(CValue));
     vsetc(type, VT_CONST, &cval);
 }
 
@@ -337,29 +336,23 @@ void vpush(CType *type)
 ST_FUNC void vpushi(int v)
 {
     CValue cval;
-    memset(&cval, 0, sizeof(CValue));
     cval.i = v;
     vsetc(&int_type, VT_CONST, &cval);
 }
 
 /* push a pointer sized constant */
-static void vpushs(long long v)
+static void vpushs(addr_t v)
 {
   CValue cval;
-  memset(&cval, 0, sizeof(CValue));
-  if (PTR_SIZE == 4)
-    cval.i = (int)v;
-  else
-    cval.ull = v;
+  cval.ptr_offset = v;
   vsetc(&size_type, VT_CONST, &cval);
 }
 
 /* push arbitrary 64bit constant */
 void vpush64(int ty, unsigned long long v)
 {
-    CType ctype;
     CValue cval;
-    memset(&cval, 0, sizeof(CValue));
+    CType ctype;
     ctype.t = ty;
     ctype.ref = NULL;
     cval.ull = v;
@@ -376,9 +369,7 @@ static inline void vpushll(long long v)
 static inline void vpushsym(CType *type, Sym *sym)
 {
     CValue cval;
-    memset(&cval, 0, sizeof(CValue));
-
-    cval.ull = 0;
+    cval.ptr_offset = 0;
     vsetc(type, VT_CONST | VT_SYM, &cval);
     vtop->sym = sym;
 }
@@ -451,7 +442,6 @@ ST_FUNC void vpush_global_sym(CType *type, int v)
 ST_FUNC void vset(CType *type, int r, int v)
 {
     CValue cval;
-    memset(&cval, 0, sizeof(CValue));
 
     cval.i = v;
     vsetc(type, r, &cval);
@@ -737,7 +727,6 @@ ST_FUNC int gv(int rc)
             unsigned long offset;
 #if defined(TCC_TARGET_ARM) && !defined(TCC_ARM_VFP)
             CValue check;
-            memset(&check, 0, sizeof(CValue));
 #endif
             
             /* XXX: unify with initializers handling ? */
@@ -770,7 +759,7 @@ ST_FUNC int gv(int rc)
             sym = get_sym_ref(&vtop->type, data_section, offset, size << 2);
             vtop->r |= VT_LVAL | VT_SYM;
             vtop->sym = sym;
-            vtop->c.ull = 0;
+            vtop->c.ptr_offset = 0;
         }
 #ifdef CONFIG_TCC_BCHECK
         if (vtop->r & VT_MUSTBOUND) 
@@ -1581,7 +1570,7 @@ static inline int is_null_pointer(SValue *p)
         return 0;
     return ((p->type.t & VT_BTYPE) == VT_INT && p->c.i == 0) ||
         ((p->type.t & VT_BTYPE) == VT_LLONG && p->c.ll == 0) ||
-	((p->type.t & VT_BTYPE) == VT_PTR && p->c.ptr == 0);
+	((p->type.t & VT_BTYPE) == VT_PTR && p->c.ptr_offset == 0);
 }
 
 static inline int is_integer_btype(int bt)
@@ -3886,8 +3875,7 @@ ST_FUNC void unary(void)
             mk_pointer(&s->type);
             s->type.t |= VT_STATIC;
         }
-        vset(&s->type, VT_CONST | VT_SYM, 0);
-        vtop->sym = s;
+        vpushsym(&s->type, s);
         next();
         break;
     
@@ -3939,7 +3927,7 @@ ST_FUNC void unary(void)
         /* if forward reference, we must point to s */
         if (vtop->r & VT_SYM) {
             vtop->sym = s;
-	    vtop->c.ull = 0;
+	    vtop->c.ptr_offset = 0;
         }
         break;
     }
@@ -5157,7 +5145,7 @@ static void init_putv(CType *type, Section *sec, unsigned long c,
             if (vtop->r & VT_SYM) {
                 greloc(sec, vtop->sym, c, R_DATA_PTR);
             }
-            *(addr_t *)ptr |= (vtop->c.ull & bit_mask) << bit_pos;
+            *(addr_t *)ptr |= (vtop->c.ptr_offset & bit_mask) << bit_pos;
             break;
         default:
             if (vtop->r & VT_SYM) {