[foreign-memaccess+abi] RFR: Fix so that heap segments work

[Per Minborg]

This PR fixes converting heap segments to strings.

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Commit messages:
 - Fix so that heap segments work

Changes: https://git.openjdk.org/panama-foreign/pull/858/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=858&range=00
  Stats: 32 lines in 2 files changed: 28 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/858.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/858/head:pull/858

PR: https://git.openjdk.org/panama-foreign/pull/858