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

x86_64-gen.c

@@ -2228,7 +2228,6 @@ ST_FUNC void gen_vla_alloc(CType *type, int align) {
     vpush_global_sym(&func_old_type, TOK_alloca);
     vswap(); /* Move alloca ref past allocation size */
     gfunc_call(1);
-    vset(type, REG_IRET, 0);
 #else
     int r;
     r = gv(RC_INT); /* allocation size */