RFR: 8367859: Remove nio exception gensrc

[Magnus Ihse Bursie]

An alternative implementation to https://github.com/openjdk/jdk/pull/27338, which removes the entire gensrc stage and instead permanently checks in the generated Java files.

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Commit messages:
 - Replace tab with spaces
 - 8367859: Remove nio exception gensrc

Changes: https://git.openjdk.org/jdk/pull/27352/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27352&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367859
  Stats: 2055 lines in 39 files changed: 1529 ins; 526 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27352.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27352/head:pull/27352

PR: https://git.openjdk.org/jdk/pull/27352