arch POSIX: Refactor into a top and bottom

Where the bottom is the only one which interacts with
the host operating system, and does not necessarily
need to know about the Zephyr OS.
This is in preparation for the native simulator,
which which the bottom is also fully Zephy agnostic.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>