[jdk16u] RFR: 8263968: CDS: java/lang/ModuleLayer.EMPTY_LAYER should be singleton

[Alexander Scherbatiy]

This is the jdk16u backport of the fix for JDK-8263968. The original patch applies cleanly without modification. Tested with JCK and jtreg.

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Commit messages:
 - Backport 133a63b4a1a2ef2e4a51fdf9edac073078692f39

Changes: https://git.openjdk.java.net/jdk16u/pull/90/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=90&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263968
  Stats: 22 lines in 3 files changed: 19 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/90.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/90/head:pull/90

PR: https://git.openjdk.java.net/jdk16u/pull/90