kernel: arch: move arch_swap() declaration

Moves the arch_swap() declaration out of kernel_arch_interface.h
and into the various architectures' kernel_arch_func.h. This
permits the arch_swap() to be inlined on ARM, but extern'd on
the other architectures that still implement arch_swap().

Inlining this function on ARM has shown at least a +5% performance
boost according to the thread_metric benchmark on the disco_l475_iot1
board.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>