aarch64: fatal: Restrict oops-es when in user-mode
aarch64: fatal: Restrict oops-es when in user-mode

User mode is only allowed to induce oopses and stack check failures via
software-triggered system fatal exceptions.

Signed-off-by: Carlo Caione <ca4506c8c654a4ccf75802e86b5d162c1e96f065@baylibre.com>

galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr