[foreign-memaccess+abi] RFR: Fix bug in hashCode after JDK-8291473 [v2]

[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).

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Add hashcode test

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/701/files
  - new: https://git.openjdk.org/panama-foreign/pull/701/files/865a40fe..a330efb4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=701&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=701&range=00-01

  Stats: 19 lines in 1 file changed: 19 ins; 0 del; 0 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