RFR: 8320767: Use := wherever possible in spec.gmk.in

Tue Nov 28 17:09:27 UTC 2023

In quite a few places we are using the late evaluation assignment `=` instead of normal assignment `:=` in spec.gmk.in.

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Commit messages:
 - 8320767: Use := for non-recursive assignment in spec.gmk.in

Changes: https://git.openjdk.org/jdk/pull/16859/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16859&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320767
  Stats: 34 lines in 1 file changed: 1 ins; 0 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/16859.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16859/head:pull/16859

PR: https://git.openjdk.org/jdk/pull/16859