kernel: activate arch interface headers

Duplicate definitions elsewhere have been removed.

A couple functions which are defined by the arch interface
to be non-inline, but were implemented inline by native_posix
and intel64, have been moved to non-inline.

Some missing conditional compilation for z_arch_irq_offload()
has been fixed, as this is an optional feature.

Some massaging of native_posix headers to get everything
in the right scope.

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