kernel: add architecture interface headers

include/sys/arch_inlines.h will contain all architecture APIs
that are used by public inline functions and macros,
with implementations deriving from include/arch/cpu.h.

kernel/include/arch_interface.h will contain everything
else, with implementations deriving from
arch/*/include/kernel_arch_func.h.

Instances of duplicate documentation for these APIs have been
removed; implementation details have been left in place.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>