kconfig: change $(ARCH_DIR) to arch/

Changing $(ARCH_DIR)/common/Kconfig to arch/common/Kconfig.

The use of ARCH_DIR at this place is wrong, as it suddenly requires out
of tree archs to support a common/Kconfig file, which may make no sense
to them.

If an out of tree arch wants to place common Kconfig code in a common
Kconfig file, that's their choice and they should source such file
themselves.

Instead just source the Zephyr arch common file directly.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>