RFR: 8358294: Remove unnecessary GenAlignment

[Albert Mingkun Yang]

Simple replacement of `GenAlignment` with `SpaceAlignment`, because they always have the same value. Removing the former to reduce complexity.

Test: tier1-3

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Commit messages:
 - remove-gen-alignment

Changes: https://git.openjdk.org/jdk/pull/25577/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25577&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358294
  Stats: 105 lines in 16 files changed: 0 ins; 46 del; 59 mod
  Patch: https://git.openjdk.org/jdk/pull/25577.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25577/head:pull/25577

PR: https://git.openjdk.org/jdk/pull/25577