arch: arc: handle exception in privilege task when USERSPACE enabled
arch: arc: handle exception in privilege task when USERSPACE enabled

when USERSPACE is enabled, exception is handled in the privilege
stack of thread. This make thread context switch is possible in the
exception handler. For some case,e.g. tests, this is useful.

Signed-off-by: Wayne Ren <d5c6d7c4aa45e53f819f17870783d153a32ba7d0@synopsys.com>

GiulianoFranchetto/zephyr,ldts/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,kraj/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,galak/zephyr,zephyriot/zephyr,zephyriot/zephyr,ldts/zephyr,ldts/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,nashif/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,galak/zephyr,galak/zephyr,punitvara/zephyr,kraj/zephyr,mbolivar/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,mbolivar/zephyr,explora26/zephyr,kraj/zephyr,mbolivar/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,zephyriot/zephyr,explora26/zephyr,galak/zephyr,kraj/zephyr,galak/zephyr,kraj/zephyr,nashif/zephyr,punitvara/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr