arch: riscv: fix sp of supervisor thread in _Fault function.
arch: riscv: fix sp of supervisor thread in _Fault function.

Although CONFIG_USERSPACE is enabled, there are supervisor threads who
don't have privileged stack using exception handler. Only let user
threads to switch to privileged stack in exception handler.

Signed-off-by: Jim Shu <1635390bb889c4cdee38f53a57b0915ac07d9bd9@andestech.com>

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