tests kernel: Exclude posix arch for userspace tests

These tests cannot be run in this architecture as it does not
support userspace.
Today they are filtered by kconfig, which works but spends
time running cmake.
As native_posix is a default test platform it is better
to filter it alltogether by arch, which saves quite a lot
of time.

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