RFR: 8373207: Make DeferredStatic class template constant initializable

[Kim Barrett]

Please review this change to `DeferredStatic` to meet the requirements for
constant initialization, to ensure dynamic initialization won't even be
considered a possibility.

Testing: mach5 tier1.

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Commit messages:
 - constexpr ctor

Changes: https://git.openjdk.org/jdk/pull/28710/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28710&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373207
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28710.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28710/head:pull/28710

PR: https://git.openjdk.org/jdk/pull/28710