[foreign-memaccess+abi] RFR: 8300201: When storing MemoryAddress.ofLong(0x0000000080000000L), MemorySegment.get is not equal to MemorySegment.set because of the expanded sign [v2]

[Per Minborg]

> This PR proposes to normalize the upper 32-bit to zero on 32-bit systems.

Per Minborg has updated the pull request incrementally with two additional commits since the last revision:

 - Update comments for 32-bit platforms
 - Update src/java.base/share/classes/java/lang/foreign/MemorySegment.java
   
   Co-authored-by: Maurizio Cimadamore <54672762+mcimadamore at users.noreply.github.com>

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/774/files
  - new: https://git.openjdk.org/panama-foreign/pull/774/files/779c0c34..292b2f4e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=774&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=774&range=00-01

  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/774.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/774/head:pull/774

PR: https://git.openjdk.org/panama-foreign/pull/774