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

tccgen.c

@@ -4033,6 +4033,7 @@ ST_FUNC void unary(void)
                 tcc_error("__builtin_va_start expects a local variable");
             vtop->r &= ~(VT_LVAL | VT_REF);
             vtop->type = char_pointer_type;
+            vtop->c.i += 8;
             vstore();
         }
         break;