cpu/arm7_common: align stacks correctly
cpu/arm7_common: align stacks correctly

`thread_stack_init()` didn't correctly set up the stack alignment.
This fixes the issue and brings the function closer to the Cortex M
version, laying the groundwork for future code duplication.

This fixes https://github.com/RIOT-OS/RIOT/issues/11885

kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT