RFR: 8345465: Fix performance regression on x64 after JDK-8345120

[Per Minborg]

This PR proposes to fix a performance regression (on x64 platforms) for quad-string words introduced by [JDK-8345120](https://bugs.openjdk.org/browse/JDK-8345120).

The PR also fixes a performance regression in the benchmarks caused by using the wrong type for `MemorySegment`.

Regrettably, this PR uses different code paths for various architectures. This gives optimum performance for all platforms at the expense of slightly more code complexity.

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Commit messages:
 - Fix regression on x64
 - Go back to use MemorySegment types
 - Merge branch 'master' into strlen-bench-update
 - Use aliases for legacy code.

Changes: https://git.openjdk.org/jdk/pull/22539/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22539&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345465
  Stats: 25 lines in 2 files changed: 5 ins; 2 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/22539.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22539/head:pull/22539

PR: https://git.openjdk.org/jdk/pull/22539