samples/philosophers: use stack_data_t type

This commit changes the pointer type used in the philosophers demo to
`stack_data_t *` when stacks are used for synchronization purposes.
The previously used `void *` was causing AMO address-misaligned
exception on the HiFive Unleashed platform.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>