kernel/include: Put kernel stack memory in the right memory

The CONFIG_KERNEL_COHERENCE framework merged with a typo that left its
validation asserts disabled.  But it was written before the "kernel
stacks" feature merged, and so missed the K_KERNEL_STACK_* macros,
which need to put their stacks into __stackmem and not merely
__noinit.

Turning the asserts on exposed the bug.

Fixes #32112

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>