[foreign-memaccess+abi] RFR: 8292851: MemorySegment arithmetics performed with int accuracy where long is required

[Per Minborg]

This PR fixes a potentially severe `int` overflow issue.

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Commit messages:
 - Use long arithmetics

Changes: https://git.openjdk.org/panama-foreign/pull/712/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=712&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292851
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/712.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/712/head:pull/712

PR: https://git.openjdk.org/panama-foreign/pull/712