x86: re-run exception-from-stub recovery selftests with CET-SS enabled

x86: re-run exception-from-stub recovery selftests with CET-SS enabled

On the BSP, shadow stacks are enabled only relatively late in the
booting process. They in particular aren't active yet when initcalls are
run. Keep the testing there, but invoke that testing a 2nd time when
shadow stacks are active, to make sure we won't regress that case after
addressing XSA-451.

While touching this code, switch the guard from NDEBUG to CONFIG_DEBUG,
such that IS_ENABLED() can validly be used at the new call site.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>