FreeRTOS: Use C11 _Static_assert to verify static "dummy" structs at compile time

FreeRTOS: Use C11 _Static_assert to verify static "dummy" structs at compile time

Includes a tweak to make Static_task_t equal size to TCB_t when using
MPU_WRAPPERS . Matches tskSTATIC_AND_DYNAMIC_ALLOCATION_POSSIBLE macro
in tasks.c. This isn't actually a bug (if static task allocation is off,
there is no use for Static_task_t), but it allows us to make consistent
compile-time checks that Static_task_t == TCB_t.