samples subsys logger: Exclude some subtests from posix arch

Two substests (userspace and rtt) cannot be run in the posix
architecture.
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>