RFR: 8329331: Intrinsify Unsafe::setMemory [v15]

[Scott Gibbons]

> This code makes an intrinsic stub for `Unsafe::setMemory` for x86_64.  See [this PR](https://github.com/openjdk/jdk/pull/16760) for discussion around this change.
> 
> Overall, making this an intrinsic improves overall performance of `Unsafe::setMemory` by up to 4x for all buffer sizes.
> 
> Tested with tier-1 (and full CI).  I've added a table of the before and after numbers for the JMH I ran (`MemorySegmentZeroUnsafe`).
> 
> [setMemoryBM.txt](https://github.com/openjdk/jdk/files/14808974/setMemoryBM.txt)

Scott Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  Change fill routines
  
  * Even more review comments
  
  * Re-write of atomic copy loops

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18555/files
  - new: https://git.openjdk.org/jdk/pull/18555/files/6e731c86..405e4e05

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18555&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18555&range=13-14

  Stats: 251 lines in 1 file changed: 103 ins; 117 del; 31 mod
  Patch: https://git.openjdk.org/jdk/pull/18555.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18555/head:pull/18555

PR: https://git.openjdk.org/jdk/pull/18555