RFR: 8308943: jdk.internal.le build fails on AIX

[Jan Lahoda]

Build of `jdk.internal.le` fails on AIX, because code in the `unix` directory expects `jdk.internal.org.jline.terminal.impl.jna.JDKNativePty` for each platform (unix-like), but it does not for AIX.

We could add a stub of this class into the `aix` directory (methods could throw an exception, which would automatically switch JLine into the "external executable" mode, if enabled). But, it might be better to use the `ServiceLoader`, and simply register a platform-specific provider as a service. This patch is attempting to do the latter.

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Commit messages:
 - 8308943: jdk.internal.le build fails on AIX

Changes: https://git.openjdk.org/jdk/pull/14176/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14176&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308943
  Stats: 323 lines in 8 files changed: 217 ins; 102 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14176.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14176/head:pull/14176

PR: https://git.openjdk.org/jdk/pull/14176