RFR: 8277617: Adjust AVX3Threshold for copy/fill stubs [v7]

[Sandhya Viswanathan]

> Currently 32-byte instructions are used for small array copy and clear. 
> This can be optimized by using 64-byte instructions.
> 
> Please review.
> 
> Best Regards,
> Sandhya

Sandhya Viswanathan has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Merge branch 'master' of https://git.openjdk.java.net/jdk into copyclearopt
 - Fix whitespace
 - Implement review comments
 - Override threshold only if flag is default
 - update comment for avx3_threshold() with more details
 - restrict to Intel core and add comment
 - 8277617: Optimize array copy and clear on x86_64

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6512/files
  - new: https://git.openjdk.java.net/jdk/pull/6512/files/190f974c..9cbfc374

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6512&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6512&range=05-06

  Stats: 34253 lines in 918 files changed: 20994 ins; 7898 del; 5361 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6512.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6512/head:pull/6512

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