RFR: making jextract to work with foreign-memaccess+abi branch of panama-foreign

[Athijegannathan Sundararajan]

making jextract to work with foreign-memaccess+abi branch of panama-foreign

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Commit messages:
 - fixed libffmpeg for library version and other merge issues
 - cherry-pick brought more changes than needed. fixed.
 - sync jextract with panama-foreign mcimadamore's "ffm_next" branch
 - changes for MemorySession.addCloseAction, null Runnable argument removal in MemorySegment.ofAddress calls,
 - using NULL instead of 0 address in tensorflow
 - Fixed samples for API changes and scripts for enable-preview source version.
 - Fix allocator issues
 - Tweak method names
 - Fix samples
 - Drop struct unsafe builder
 - ... and 3 more: https://git.openjdk.org/jextract/compare/0582eaf1...32b7cec3

Changes: https://git.openjdk.org/jextract/pull/59/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=59&range=00
  Stats: 750 lines in 114 files changed: 34 ins; 156 del; 560 mod
  Patch: https://git.openjdk.org/jextract/pull/59.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/59/head:pull/59

PR: https://git.openjdk.org/jextract/pull/59