arch: arm: Fix compile error on ARMv6-M SoCs with TICKLESS_KERNEL

pop {lr} instruction is not supported in ARMv6-M, fixed by
using pop {r0}; mov lr, r0; instructions.

Jira: ZEP-2222

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>