win64: fix va_arg

fixes 5c35ba66c5

Implementation was consistent within tcc but incompatible
with the ABI (for example library functions vprintf etc)

Also:
- tccpp.c/get_tok_str() : avoid "unknown format "%llu" warning
- x86_64_gen.c/gen_vla_alloc() : fix vstack leak

tccpp.c

@@ -464,9 +464,6 @@ ST_FUNC const char *get_tok_str(int v, CValue *cv)
     switch(v) {
     case TOK_CINT:
     case TOK_CUINT:
-        /* XXX: not quite exact, but only useful for testing */
-        sprintf(p, "%llu", (unsigned long long)cv->i);
-        break;
     case TOK_CLLONG:
     case TOK_CULLONG:
         /* XXX: not quite exact, but only useful for testing  */