arm: swap: cleanup an #ifdef statement in swap routine

Cleanup an #ifdef statement in swap_helper.S; use
ARMV6_M_ARMV8_M_BASELINE instead of listing all
Cortex-M baseline implementation variants. This
fixes an issue with Cortex-M23 whose Kconfig
define was not included in the original list.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>