RFR: 8336938: Update libFFI to 3.4.6

[Alexander Matveev]

- libFFI updated to 3.4.6.
- No additional changes are done.
- Tested on Windows, macOS and Linux with all supported formats.

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into JDK-8336938
 - 8336938: Update libFFI to 3.4.6

Changes: https://git.openjdk.org/jfx/pull/1531/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1531&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336938
  Stats: 1055 lines in 26 files changed: 388 ins; 373 del; 294 mod
  Patch: https://git.openjdk.org/jfx/pull/1531.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1531/head:pull/1531

PR: https://git.openjdk.org/jfx/pull/1531