Fix access to the DWT peripheral for STM32 HAL (#12434)

Access to the DWT peripheral for the `CYCCNT` register needs to happen before `main()`. The code needs to be called after the setup of the system clocks, so the right place is between the `premain()` and `main()` function of the STM32 Arduino core.

This patch moves the DWT access code to a new function, which is then placed between `premain()` and `main()`.