kernel: dynamic: declare dynamic stubs when disabled

With some of the recent work to disable unnecessary system
calls, there is a scenario where `z_impl_k_thread_stack_free()`
is not defined and an undefined symbol error occurs.

Safety was very concerned that dynamic thread stack code might
touch other code that does not malloc, so add a separate file
for the stack alloc and free stubs.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>