8154495: SHA256 AVX2 intrinsic (when no supports_sha() available)

[Civlin, Jan]

We would like to contribute the SHA256 AVX2 intrinsic.

This intrinsic is for x86 AVX2 architecture when no supports_sha() is available. It is L64 code only.

The patch delivers x2 performance gain for the latency and throughput - both are measured on an average message.  

Contributor: Jan Civlin.


bug: https://bugs.openjdk.java.net/browse/JDK-8154495
webrev: http://cr.openjdk.java.net/~vdeshpande/8154495/webrev.00/