[foreign-jextract] RFR: Fix issues caused by removal of the Foreign class

[Maurizio Cimadamore]

This patch rewires jextract to use the new MemorySegment::ofNativeRestricted method.

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Commit messages:
 - Fix issues caused by removal of the Foreign class

Changes: https://git.openjdk.java.net/panama-foreign/pull/129/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/129/webrev.00
  Stats: 47 lines in 21 files changed: 5 ins; 10 del; 32 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/129.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/129/head:pull/129

PR: https://git.openjdk.java.net/panama-foreign/pull/129