RFR: 8277617: Adjust AVX3Threshold for copy/fill stubs

[Sandhya Viswanathan]

On Tue, 23 Nov 2021 06:49:07 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> @dholmes-ora I have implemented your review comments.
>
> Sorry @sviswa7 but could you explain in the comment why/how `avx3_threshold` reporting zero impacts the use 64-byte load/store - the connection is not at all obvious for anyone not fully conversant with AVX3 and how it is used by the code. Thanks.

@dholmes-ora  @neliasso Please do approve the patch if it looks ok to you.

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PR: https://git.openjdk.java.net/jdk/pull/6512