[lworld] RFR: 8378767: [lworld] Rename Makefile functions and variables related to preview mode

[David Beaumont]

Renames value class related functions and variables in Makefiles to reflect the more general use for preview mode classes/resources (where appropriate).

As preview mode is enabled in mainline builds, it will no longer be limited to value classes and their semantics.

After this change, the work "value" appears only once in the modified files (outside of comments) when defining GENERATED_PREVIEW_SUBDIRS.

This does change the `_the.$(MODULE).valueclasses` target to be `_the.$(MODULE).preview` but that should not be an issue.

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Commit messages:
 - switch nomenclature from value class to preview

Changes: https://git.openjdk.org/valhalla/pull/2175/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2175&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378767
  Stats: 33 lines in 2 files changed: 6 ins; 0 del; 27 mod
  Patch: https://git.openjdk.org/valhalla/pull/2175.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2175/head:pull/2175

PR: https://git.openjdk.org/valhalla/pull/2175