[foreign-memaccess+abi] RFR: 8313238: Improve the performance of strlen for null-terminated strings

[Per Minborg]

This PR proposes to use native calls for determining the string length for C-type strings. 

Smaller segments (<1024 bytes) is using a trivial call whereas all other segments are using a normal call. We might consider always using regular calls if we think most segments are unbound anyhow.

The PR also contains a number of new tests, one of which requires a large heap size to run. It is likely that this latter test will not run in most test environments. In such cases, it is silently ignored.

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Commit messages:
 - Clean up
 - Use native call for strlen
 - Use long access

Changes: https://git.openjdk.org/panama-foreign/pull/857/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=857&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313238
  Stats: 285 lines in 2 files changed: 264 ins; 4 del; 17 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/857.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/857/head:pull/857

PR: https://git.openjdk.org/panama-foreign/pull/857