[foreign-jextract] [Rev 01] 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

Athijegannathan Sundararajan has updated the pull request incrementally with one additional commit since the last
revision:

  renamed upcall stub factory method "$make" as "allocate"

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/138/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/138/files/eedbc342..f8849927

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/138/webrev.01
 - incr: https://webrevs.openjdk.java.net/panama-foreign/138/webrev.00-01

  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 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