[foreign-jextract] RFR: 8244172: jextract should generate upcall stub creating helper in the functional interface class

[Athijegannathan Sundararajan]

* moved the upcall stub creating utility method as static method of functional interface
* added free upcall stub utility to RuntimeHelper
* added a simple test for function pointer callback

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Commit messages:
 - fixed the shared library name in the test.
 - 8244172: jextract should generate upcall stub creating helper in the functional interface class

Changes: https://git.openjdk.java.net/panama-foreign/pull/138/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/138/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244172
  Stats: 147 lines in 8 files changed: 127 ins; 16 del; 4 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/138.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/138/head:pull/138

PR: https://git.openjdk.java.net/panama-foreign/pull/138