kernel: fatal: use nested exception info in z_fatal_error

In z_fatal_error() we invoke the arch-specific API that
evaluates whether we are in a nested exception. We then
use the result to log a message that the error occurred
in ISR. In non-test mode, we unconditionally panic, if
an exception has occurred in an ISR and the fatal error
handler has not returned (apart from the case of an
error in stack sentinel check).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>