RFR: 8345465: Fix performance regression on x64 after JDK-8345120 [v2]

[Per Minborg]

> This PR proposes to fix a performance regression (on x64 platforms) for 32-bit strings 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.

Per Minborg has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

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

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22539/files
  - new: https://git.openjdk.org/jdk/pull/22539/files/779b7f91..4d41488b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22539&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22539&range=00-01

  Stats: 4069 lines in 152 files changed: 2351 ins; 1020 del; 698 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