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).

tcc.h

@@ -55,18 +55,27 @@
 # ifndef CONFIG_TCC_STATIC
 #  include <dlfcn.h>
 # endif
-#else
+/* XXX: need to define this to use them in non ISOC99 context */
+ extern float strtof (const char *__nptr, char **__endptr);
+ extern long double strtold (const char *__nptr, char **__endptr);
+#else /* on _WIN32: */
 # include <windows.h>
 # include <sys/timeb.h>
 # include <io.h> /* open, close etc. */
 # include <direct.h> /* getcwd */
 # ifdef __GNUC__
 #  include <stdint.h>
-# else
-   typedef UINT_PTR uintptr_t;
 # endif
 # define inline __inline
 # define inp next_inp
+# define snprintf _snprintf
+# define vsnprintf _vsnprintf
+# ifndef __GNUC__
+#  define strtold (long double)strtod
+#  define strtof (float)strtod
+#  define strtoll _strtoi64
+#  define strtoull _strtoui64
+# endif
 # ifdef LIBTCC_AS_DLL
 #  define LIBTCCAPI __declspec(dllexport)
 #  define PUB_FUNC LIBTCCAPI
@@ -79,6 +88,30 @@
 # define O_BINARY 0
 #endif
 
+#ifdef __GNUC__
+# define NORETURN __attribute__ ((noreturn))
+#elif defined _MSC_VER
+# define NORETURN __declspec(noreturn)
+#else
+# define NORETURN
+#endif
+
+#ifdef _WIN32
+# define IS_DIRSEP(c) (c == '/' || c == '\\')
+# define IS_ABSPATH(p) (IS_DIRSEP(p[0]) || (p[0] && p[1] == ':' && IS_DIRSEP(p[2])))
+# define PATHCMP stricmp
+#else
+# define IS_DIRSEP(c) (c == '/')
+# define IS_ABSPATH(p) IS_DIRSEP(p[0])
+# define PATHCMP strcmp
+#endif
+
+#ifdef TCC_TARGET_PE
+#define PATHSEP ';'
+#else
+#define PATHSEP ':'
+#endif
+
 #include "elf.h"
 #ifdef TCC_TARGET_X86_64
 # define ELFCLASSW ELFCLASS64
@@ -315,7 +348,7 @@ typedef union CValue {
     long long ll;
     unsigned long long ull;
     struct CString *cstr;
-    void *ptr;
+    addr_t ptr_offset;
     int tab[LDOUBLE_SIZE/4];
 } CValue;
 
@@ -938,37 +971,6 @@ enum tcc_token {
 
 #define TOK_UIDENT TOK_DEFINE
 
-#ifdef _WIN32
-#define snprintf _snprintf
-#define vsnprintf _vsnprintf
-#ifndef __GNUC__
-# define strtold (long double)strtod
-# define strtof (float)strtod
-# define strtoll _strtoi64
-# define strtoull _strtoui64
-#endif
-#else
-/* XXX: need to define this to use them in non ISOC99 context */
-extern float strtof (const char *__nptr, char **__endptr);
-extern long double strtold (const char *__nptr, char **__endptr);
-#endif
-
-#ifdef _WIN32
-#define IS_DIRSEP(c) (c == '/' || c == '\\')
-#define IS_ABSPATH(p) (IS_DIRSEP(p[0]) || (p[0] && p[1] == ':' && IS_DIRSEP(p[2])))
-#define PATHCMP stricmp
-#else
-#define IS_DIRSEP(c) (c == '/')
-#define IS_ABSPATH(p) IS_DIRSEP(p[0])
-#define PATHCMP strcmp
-#endif
-
-#ifdef TCC_TARGET_PE
-#define PATHSEP ';'
-#else
-#define PATHSEP ':'
-#endif
-
 /* space exlcuding newline */
 static inline int is_space(int ch)
 {
@@ -1045,7 +1047,7 @@ PUB_FUNC char *tcc_strdup(const char *str);
 #define strdup(s) use_tcc_strdup(s)
 PUB_FUNC void tcc_memstats(void);
 PUB_FUNC void tcc_error_noabort(const char *fmt, ...);
-PUB_FUNC void tcc_error(const char *fmt, ...) __attribute__ ((noreturn));
+PUB_FUNC NORETURN void tcc_error(const char *fmt, ...);
 PUB_FUNC void tcc_warning(const char *fmt, ...);
 
 /* other utilities */
@@ -1143,7 +1145,7 @@ ST_FUNC void preprocess_init(TCCState *s1);
 ST_FUNC void preprocess_new(void);
 ST_FUNC int tcc_preprocess(TCCState *s1);
 ST_FUNC void skip(int c);
-ST_FUNC void expect(const char *msg) __attribute__ ((noreturn));
+ST_FUNC NORETURN void expect(const char *msg);
 
 /* ------------ tccgen.c ------------ */