kernel: stacks: add k_stack_alloc() init

Similar to what has been done with pipes and message queues,
user mode can't be trusted to provide a buffer for the kernel
to use. Remove k_stack_init() as a syscall and offer
k_stack_alloc_init() which allocates a buffer from the caller's
resource pool.

Fixes #7285

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>