RFR: 8330185: Potential uncaught unsafe memory copy exception

[Scott Gibbons]

Adding an `UnsafeCopyMemoryMark` in `generate_disjoint_copy_avx3_masked()` to protect against SIGBUS in `arraycopy_avx3_large()`.  I discovered this by code inspection, and the missing memory mark is inconsistent with all other generators.  I do not have a testcase to generate such an exception.  I think this may have been a copy/paste error by the original contributor as evidenced by the variable `ucme_exit_pc` having been set but never used.

I have not seen a VM crash that can be attributed to this, but adding the mark is the correct behavior for its prevention.

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Commit messages:
 - Add UnsafeCopyMemoryMark around arraycopy_avx3_large()

Changes: https://git.openjdk.org/jdk/pull/18766/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18766&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330185
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18766.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18766/head:pull/18766

PR: https://git.openjdk.org/jdk/pull/18766