RFR: 8322309: Fix an inconsistancy in spacing style in spec.gmk.template

[Frederic Thevenet]

[JDK-8320763](https://bugs.openjdk.org/browse/JDK-8320763) fixed the consistency of spaces around the assignment operators in spec.gmk.in (as it was called then).
Unfortunately [JDK-8321374](https://bugs.openjdk.org/browse/JDK-8321374) re-introduced one such inconsistency and so this follow-up aims to fix this.

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Commit messages:
 - Fix an inconsistancy in spacing style in spec.gmk.template

Changes: https://git.openjdk.org/jdk/pull/17149/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17149&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322309
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17149.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17149/head:pull/17149

PR: https://git.openjdk.org/jdk/pull/17149