Flag -XX:AVX3Threshold does not accept Zero value

AVX3Threshold has been used in various places to enable emitting AVX3 specific instructions in case data size being worked over is greater than 4096 bytes.

However, user is free to set the threshold value to Zero based on his workload.
In such a case a compile time check is enough to trigger generation of AVX3 instructions. In other cases comparison is done at the runtime though JITed comparison instruction.

AVX3Threshold does not accept a zero value.