[foreign-jextract] RFR: 8251049: revisit address accessors for struct fields/global variables

[Athijegannathan Sundararajan]

* Non-cloneable, non-transferable segments are returned now
* $ADDR/$addr methods renamed as $SEGMENT/$slice respectively
* Existing test is modified to add check for segment access mode

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Commit messages:
 - 8251049: revisit address accessors for struct fields/global variables

Changes: https://git.openjdk.java.net/panama-foreign/pull/282/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/282/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251049
  Stats: 53 lines in 8 files changed: 13 ins; 0 del; 40 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/282.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/282/head:pull/282

PR: https://git.openjdk.java.net/panama-foreign/pull/282