libC kconfig: default to PICOLIBC for NATIVE_LIBRARY if POSIX_API

When building with the POSIX_API we cannot use the host libC.
When using GETOPT it is similarly quite difficult for users
to use the host libC.
So to make it easier for users, let's just default to PICOBLIC
in those cases,
while we continue defaulting to the host library in other cases
so users can use the Linux APIs for whatever test functionality
they want.

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