[foreign-jextract] RFR: 8253419: jextract should generate RuntimeHelper as a package private class

[Athijegannathan Sundararajan]

Generated RuntimeHelper.java is package private now.

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Commit messages:
 - 8253419: jextract should generate RuntimeHelper as a package private class

Changes: https://git.openjdk.java.net/panama-foreign/pull/347/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=347&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253419
  Stats: 386 lines in 9 files changed: 14 ins; 179 del; 193 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/347.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/347/head:pull/347

PR: https://git.openjdk.java.net/panama-foreign/pull/347