lib POSIX: Enable selectively for POSIX ARCH

The POSIX API compatibility shim can be used for some
of the POSIX ARCH targets.
Narrow the Kconfig filtering accordingly.

Note that the recommended configuration when building
with the native simulator is still to use an embedded
C library. Using the host C library will in some cases
cause undesired behaviour.

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