RFR: 8305461: [vectorapi] Add VectorMask::xor

[Quan Anh Mai]

Hi,

This patch adds `VectorMask.xor` to the public interface of `VectorMask`. This method has already been existed in each concrete mask classes. Also, this patch renames `VectorMask.eq` to `VectorMask::xorNot`, which is more consistent with other logical operations in the same interface.

Please kindly review.
Thanks a lot.

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Commit messages:
 - rename xorNot back to eq
 - revert renaming eq
 - tests
 - remove dangling docs
 - add final
 - maskxor

Changes: https://git.openjdk.org/jdk/pull/13277/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13277&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305461
  Stats: 2732 lines in 64 files changed: 2411 ins; 281 del; 40 mod
  Patch: https://git.openjdk.org/jdk/pull/13277.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13277/head:pull/13277

PR: https://git.openjdk.org/jdk/pull/13277