[foreign-memaccess+abi] RFR: 8313238: Improve the performance of strlen for null-terminated strings [v3]

[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.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Break out native invocations in separate methods

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/857/files
  - new: https://git.openjdk.org/panama-foreign/pull/857/files/d3e8a26a..2790ddb2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=857&range=02
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=857&range=01-02

  Stats: 72 lines in 1 file changed: 45 ins; 7 del; 20 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