[foreign-jextract] RFR: 8243485: adjust panama_jextract.md samples after recent foreign-abi changes

[Athijegannathan Sundararajan]

removed references to Foreign class. Changed system property in scripts.

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Commit messages:
 - 8243485: adjust panama_jextract.md samples after recent foreign-abi changes

Changes: https://git.openjdk.java.net/panama-foreign/pull/130/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/130/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8243485
  Stats: 93 lines in 2 files changed: 2 ins; 6 del; 85 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/130.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/130/head:pull/130

PR: https://git.openjdk.java.net/panama-foreign/pull/130