[lworld] RFR: 8369922: [lworld] Preview class sources in the source bundle

[David Beaumont]

Adding a rule to include the value class generated sources into src.zip.

This is mostly based on work by Christian Stein, which I adapted after digging through to understand the approach taken in this Make file.

Since it took me a non-trivial amount of time to fully grok this makefile, I added a significant amount of documentation for the next person.

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Commit messages:
 - 8369922: [lworld] Preview class sources in the source bundle

Changes: https://git.openjdk.org/valhalla/pull/2132/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2132&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369922
  Stats: 56 lines in 1 file changed: 53 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/valhalla/pull/2132.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2132/head:pull/2132

PR: https://git.openjdk.org/valhalla/pull/2132