RFR: 8307527: MacOS Zero builds fail with undefined FFI_GO_CLOSURES after JDK-8304265

[Aleksey Shipilev]

See the bug. Actually, I am not sure why JDK-8304265 changed the `#ifndef FFI_GO_CLOSURES` to `#ifdef _APPLE_`. That seems too intrusive if `FFI_GO_CLOSURES` *is* enabled. So I rewrote the block to something more safe.

Additional testing:
 - [x] macos-aarch64-zero-fastdebug `make images` passes

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/13827/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13827&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307527
  Stats: 7 lines in 2 files changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13827.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13827/head:pull/13827

PR: https://git.openjdk.org/jdk/pull/13827