[code-reflection] Integrated: Synchronizing all the ffi backend native code

[Gary Frost]

The ongoing refactor of ffi backends continues. 

Moved common ffi code to ffi-shared (for all backends) 

Also changed the java backend so that they all used the same mh references.

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Commit messages:
 - Synchronizing all the ffi backend native code

Changes: https://git.openjdk.org/babylon/pull/379/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=379&range=00
  Stats: 890 lines in 24 files changed: 371 ins; 409 del; 110 mod
  Patch: https://git.openjdk.org/babylon/pull/379.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/379/head:pull/379

PR: https://git.openjdk.org/babylon/pull/379