build: downgrade the no optimization ztest error to warning

The current approach of failing the build on ztest with no optimization
broke coverage builds, and generally raised some concerns about being
too aggressive.

Downgrade the error to a warning and rework the option to inhibit the
warning, while also dropping it automatically for POSIX (that are not
really affected by stack size) and coverage run (that always runs with
no optimization).

Will reconsider this down the road if we still see issues filed for the
tests broken with no optimization and no further tuning.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>