tests newlib: Exclude in posix architecture

These tests cannot be run in this architecture as it does not
support this toolchain.
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>