kernel: assert no spinlock is held on swap when !USE_SWITCH

The do_swap() routine used when CONFIG_USE_SWITCH=y asserts that caller
thread does not hold any spinlock when CONFIG_SPIN_VALIDATE is enabled.
However, there is no similar check in place when CONFIG_USE_SWITCH=n.

Copy this assertion in the USE_SWITCH=n implementation of z_swap_irqlock().

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>