twister: native_posix: Ignor filtering by toolchain for native_posix

In the case of native_posix (and other "host" toolchain based
platforms) Twister shouldn't filter tests by ZEPHYR_TOOLCHAIN_VARIANT.
The PR replaces `- zephyr` with `- host` in native_posix(_64).yaml
and modify the filter in twisterlib.py to not block building
if "host" is under the platform supported toolchains.

Fixes: #38418

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>