RFR: 8329331: Intrinsify Unsafe::setMemory [v17]

[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 three additional commits since the last revision:

 - Set memory test (#22)
   
   * Even more review comments
   
   * Re-write of atomic copy loops
   
   * Change name of UnsafeCopyMemory{,Mark} to UnsafeMemory{Access,Mark}
   
   * Only add a memory mark for byte unaligned fill
 - Set memory test (#21)
   
   * Even more review comments
   
   * Re-write of atomic copy loops
   
   * Change name of UnsafeCopyMemory{,Mark} to UnsafeMemory{Access,Mark}
   
   * Only add a memory mark for byte unaligned fill
 - Only add a memory mark for byte unaligned fill
   
   * Even more review comments
   
   * Re-write of atomic copy loops
   
   * Change name of UnsafeCopyMemory{,Mark} to UnsafeMemory{Access,Mark}
   
   * Only add a memory mark for byte unaligned fill

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18555/files
  - new: https://git.openjdk.org/jdk/pull/18555/files/95b0a345..80b5a0ca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18555&range=16
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18555&range=15-16

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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