[lworld] RFR: 8373987: [lworld] exploded-image/test broken since disable patching (JDK-8373806)

[David Beaumont]

Preview mode support for exploded build.

This is an alternate approach from https://github.com/openjdk/valhalla/pull/1845 which I think is actually cleaner and more robust.

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Commit messages:
 - remove redundant function definition
 - Alternate preview mode support for exploded build

Changes: https://git.openjdk.org/valhalla/pull/1854/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1854&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373987
  Stats: 55 lines in 3 files changed: 25 ins; 11 del; 19 mod
  Patch: https://git.openjdk.org/valhalla/pull/1854.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1854/head:pull/1854

PR: https://git.openjdk.org/valhalla/pull/1854