Disable validateWithReference for skylake_avx512

An argument is added to validateWithReference that allows us to disable the validator based on the platform. By default only skylake_avx512 is disabled, which reflects the fact that we don't try to maintain references there (as we don't plan to use it in production).

Note that this is a "soft" disabling compared to using unsupported_platforms in the qmt file. With the latter the test is not run at all, whereas with the change here only the referece validation is disabled.

Once we have a better framework to compare counters, we can add checks on skylake where we set appropriate thresholds for the differences with respect to the references.