arm: thread_abort: clarify what's going on
arm: thread_abort: clarify what's going on

A check was being done that was a more obscure way of
calling arch_is_in_isr(). Add a comment explaining
why we need to trigger PendSV.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

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