rtos/zephyr: arm: fetch arm exc return offset

Since zephyrproject-rtos/zephyr@c3eeae8,
Zephyr OS exposes offset of mode_exc_return in the arch struct for ARM.

Accounting for this allows for consistency and enables
logic with further offsets that may be added after this.

Signed-off-by: Bruno Mendes <bd_mendes@outlook.com>
Change-Id: Id53ebd80c5d98a7d94eb6b00ad638ce51e719822
Reviewed-on: https://review.openocd.org/c/openocd/+/7851
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

src/rtos/zephyr.c

@@ -57,6 +57,7 @@ enum zephyr_offsets {
 	OFFSET_T_ARCH,
 	OFFSET_T_PREEMPT_FLOAT,
 	OFFSET_T_COOP_FLOAT,
+	OFFSET_T_ARM_EXC_RETURN,
 	OFFSET_MAX
 };