cpu/cortex-mX_common: bugfix to arch_stack_init

I think I found a quite serious bug introduced by me last week: The void pointer cast in the beginning of `thread_arch_stack_init()` computes (at least for `arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.8.3 20131129 (release) [ARM/embedded-4_8-branch revision 205641]`) a wrong `stk` variable:
```c
void *stack_start = 1000;  // decimal values for easier readablility
int stadk_size = 100; // in bytes, right?

uint32_t *stk = ((uint32_t *)stack_start + stack_size); // stk := 1400 -> wrong!
uint32_t *stk = ((uint32_t)stack_start + stack_size); // stk := 1100 -> correct
```

Does this make sense or do I have some wrinkled thoughts here?