[foreign-memaccess+abi] RFR: Improve strlen performance [v5]

[Per Minborg]

> This PR suggests removing the use of native calls for strlen and instead use Java implementations.
> 
> The PR also suggest performance improvements for quad word strlen.
> 
> Here are some benchmarks that compares the performance of the new proposed methods with the performance of the JDK 21 variants (called "legacy" methods):
> 
> <img width="2596" alt="image" src="https://github.com/openjdk/panama-foreign/assets/7457876/c785f341-c826-4e3e-bf56-8387b1e96010">

Per Minborg has updated the pull request incrementally with two additional commits since the last revision:

 - Allow unaligned access and harden tests
 - Simplify names

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/862/files
  - new: https://git.openjdk.org/panama-foreign/pull/862/files/f337376b..4a178d45

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=862&range=04
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=862&range=03-04

  Stats: 162 lines in 2 files changed: 79 ins; 17 del; 66 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/862.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/862/head:pull/862

PR: https://git.openjdk.org/panama-foreign/pull/862