RFR: 8339627: Cleanup Unsafe.setMemory intrinsic code

[Johan Sjölen]

Hi,

The code for the `Unsafe.setMemory` intrinsic has a few issues that this PR cleans up.

1. The labels are unused in x86-64 intrinsic
2. The function stub has an incorrect function prototype as it clearly manipulates the array so the array is not const, and we don't read the array so it probably shouldn't be called `src`. That's probably just an issue of `UnsafeArrayCopyStub` being copied and altered insufficiently.

Thanks.

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Commit messages:
 - Call dest dst since ArrayCopy above does so
 - Dead labels
 - src is not const void* and should be called dest

Changes: https://git.openjdk.org/jdk/pull/20873/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20873&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339627
  Stats: 4 lines in 2 files changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20873.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20873/head:pull/20873

PR: https://git.openjdk.org/jdk/pull/20873