kernel: Add kswap.h header to unbreak cycles

The xtensa-asm2 work included a patch that added nano_internal.h
includes in lots of places that needed to have _Swap defined, because
it had to break a cycle and this no longer got pulled in from the arch
headers.

Unfortunately those new includes created new and more amusing cycles
elsewhere which led to breakage on other platforms.

Break out the _Swap definition (only) into a separate header and use
that instead.  Cleaner.  Seems not to have any more hidden gotchas.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>