kernel: Reset the switch_handler only in the arch code

Avoid setting the switch_handler in the z_get_next_switch_handle() code
when the context is not fully saved yet to avoid a race against other
cores waiting on wait_for_switch().

See issue #40795 and discussion in #41840

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
2 files changed