[lworld] RFR: 8377101: [lworld] Automatically generate value class sources based on source annotations

[David Beaumont]

Adds a new AnnotationProcessor to read the @MigratedValueClass on prospective value classes and generate the equivalent source file with the 'value' keyword at each annotated class declaration.

This adds new PROCESSOR_PATH variable to the compilation macro and use it for annotation processing.
This also requires moving the plugin discovery path to the new variable (was in classpath before) because the addition of a '--processor-path' flag disables using the classpath for plugin discovery.

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Commit messages:
 - Use annotation processor to generate value class sources

Changes: https://git.openjdk.org/valhalla/pull/2180/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2180&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377101
  Stats: 309 lines in 4 files changed: 307 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/2180.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2180/head:pull/2180

PR: https://git.openjdk.org/valhalla/pull/2180