RFR(XS): 8238596: AVX enabled by default for Skylake even when unsupported

[David Buck]

Hi!

May I please get a review of this small fix?

bug report: https://bugs.openjdk.java.net/browse/JDK-8238596
webrev: http://cr.openjdk.java.net/~dbuck/8238596.0/

Efficacy of fix has been confirmed manually in VirtualBox.

Cheers,
-Buck