RFR: 8187520: Add --disable-javac-warnings-as-errors configure option

[Magnus Ihse Bursie]

Analogous with how native warnings can be treated as errors or not using the `--disable-warnings-as-errors` configure flag, this patch introduces a `--disable-javac-warnings-as-errors` flag.

The reason that this is not made part of `--disable-warnings-as-errors` is discussed in the JBS issue, but in short, native warnings can easily occur due to different build environment or compiler versions, while javac warnings are platform independent and should only depend on the source code we compile. That means that this option is basically only needed by developers to temporary silence javac while developing new code.

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Commit messages:
 - 8187520: Add --disable-javac-warnings-as-errors configure option

Changes: https://git.openjdk.org/jdk/pull/24486/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24486&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8187520
  Stats: 29 lines in 5 files changed: 20 ins; 7 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24486.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24486/head:pull/24486

PR: https://git.openjdk.org/jdk/pull/24486