kernel: Option to allow shadow stack to be reused

It seems that, at least on tests, it's common to call k_thread_create()
on a thread multiple times. This trips a check for the CET shadow stack
- namely, set a shadow stack on a thread which already has a shadow
stack.

This patch adds a Kconfig option to allow that, iff the base address and
size of the new shadow stack are the same as before. This will trigger a
reset of the shadow stack, so it can be reused.

It may be the case that this behaviour (reusing threads) is more common
than only for tests, in which case it could make sense to change the
default - in this patch, is only true if ZTEST.

Even if being enabled by default becomes the reality, it would still
make sense to keep this option - more conscious apps could avoid the
need for the shadow stack reset code altogether.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>