[lworld] RFR: 8368333: [lworld] Add preview mode to ImageReader and JRT file-system

[David Beaumont]

Java changes for supporting preview mode when preview mode resources (with new location flags) are available.

At the moment, this code will operate on non-preview jimage files (1.0) and act as if no preview resources are available by virtue of the default value for missing attributes and package flags being zero (which matches jimage 1.0).

This should be reviewed on top of https://github.com/openjdk/valhalla/pull/1618

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Commit messages:
 - Word smithing flags definitions.
 - Add workaround until new image writing code is in
 - Clarifying flag docs for /packages/xxx case
 - Java ImageReader changes for preview mode
 - C++ preparation for preview mode

Changes: https://git.openjdk.org/valhalla/pull/1619/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1619&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368333
  Stats: 1705 lines in 25 files changed: 1342 ins; 167 del; 196 mod
  Patch: https://git.openjdk.org/valhalla/pull/1619.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1619/head:pull/1619

PR: https://git.openjdk.org/valhalla/pull/1619