tests/posix/fs: Improve filtering around posix arch

These tests cannot be run on the native_posix(_64) targets,
so let's filter them explicitly. Before they were filtered
due to the "filter", but using platform_exclude is faster.

Also the tests that depend on newlib cannot be run
in this architecture at all, so let's filter them by
architecture too.

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