cpu/stm32f4: optimized GPIO driver implementation

And the same game for the STM32F4, results (for `tests/periph_gpio_int`):

before:

```
   text    data     bss     dec     hex
  13364     112   14216   27692    6c2c
```

and after:

```
   text    data     bss     dec     hex
  12484     112   14216   26812    68bc
```
GPIO_15_PIN