[foreign-memaccess+abi] RFR: Remove dead code, add tests and fixes

[Per Minborg]

This PR suggests removing dead code and addition of some tests. Also, a fix for zero-length mapped segment is added. It is anticipated this special zero-length segment is going to be removed in a separate PR.

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Commit messages:
 - Remove dead code, add tests and fixes

Changes: https://git.openjdk.org/panama-foreign/pull/792/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=792&range=00
  Stats: 132 lines in 5 files changed: 88 ins; 37 del; 7 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/792.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/792/head:pull/792

PR: https://git.openjdk.org/panama-foreign/pull/792