[foreign-memaccess+abi] RFR: Fix bug in hashCode after JDK-8291473

[Maurizio Cimadamore]

There is an asymmetry between MemorySegment::equals and MemorySegment::hashCode after the unification of memory segment and memory address.

Both methods should only take into account unsafe offset and base (e.g. two segments are equals if they point to same memory location).

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/701/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=701&range=00
  Stats: 7 lines in 1 file changed: 0 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/701.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/701/head:pull/701

PR: https://git.openjdk.org/panama-foreign/pull/701