debug: thread_info: store arch mode_exc_return offset

Store the offset of mode_exc_return in the arch struct. This is required
to restore the link register to the original value, as `swap_helper.S`
saves the LSB to this field when `CONFIG_ARM_STORE_EXC_RETURN=y`.

Failing to account for this results in broken debugging when
`FPU_SHARING` or `ARM_NONSECURE_PREEMPTIBLE_SECURE_CALLS`.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>