arch/x86: Verify shadow stack pointers

Currently, it's permitted to have threads that don't have a shadow
stack. When those are run, shadow stack is disabled on the CPU. To
identify those, the thread `shstk_addr` member is checked.

This patch adds an optional check, behind
CONFIG_X86_CET_VERIFY_KERNEL_SHADOW_STACK, that checks if an outgoing
thread has this pointer NULL with shadow stack currently enabled on
the CPU, meaning a 1) bug or 2) some attempt to tamper with the pointer.

If the check fails, k_panic() is called. Note that this verification is
not enough to guarantee `shstk_addr` can't be tampered with. For
instance, it only works on a running thread. Ideally, all threads should
be shadow stack capable, so a missing `shstk_addr` would simply be a
hard fault, but that is still to come.

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