[lworld] RFR: 8368475: [lworld] Add preview classes to jimage at make time

[David Beaumont]

Copies valuetype classes for each module into corresponding "/META-INF/preview/..." sub-directories to be pulled into jimage files and processed by the new preview mode handling code.

There might be a better way to do this in terms of Makefile semantics, but this seems to work well enough and doesn't prevent the value-class JAR files being generated for patching (which is still how everyone will get value classes until the rest of the work is plumbed in).

To enable the new preview mode work, set the "DISABLE_PREVIEW_PATCHING" system property to "true".

This should be review against https://github.com/openjdk/valhalla/pull/1621.

-------------

Commit messages:
 - Copy value classes into preview directories for inclusion in jimage
 - add system property guard to preview mode
 - jimage writer changes to support preview mode.
 - 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/1622/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1622&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368475
  Stats: 3586 lines in 34 files changed: 2112 ins; 279 del; 1195 mod
  Patch: https://git.openjdk.org/valhalla/pull/1622.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1622/head:pull/1622

PR: https://git.openjdk.org/valhalla/pull/1622