RFR: 8368475: [lworld] Add preview classes to jimage at make time [v4]

[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.

David Beaumont has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Copy value classes into preview directories for inclusion in jimage
 - Restoring lost changes and updating some comments.
 - add system property guard to preview mode
 - jimage writer changes to support preview mode.
   
   * Remove TODOs now jimage version is bumped
   * jimage writer changes to support preview mode.
 - Manually deleting ImageReaderFactory (it returned somehow)

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Changes: https://git.openjdk.org/valhalla/pull/1622/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1622&range=03
  Stats: 2004 lines in 15 files changed: 753 ins; 240 del; 1011 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