RFR: 8332744: [REDO] 'internal proprietary API' diagnostics if --system is configured to an earlier JDK version

[Liam Miller-Cushon]

This change fixes a bug preventing javac from emitting 'compiler.warn.sun.proprietary' diagnostics if `--system` is set to a non-default value. The diagnostics are currently emitted for values of `--release`, and for the default value of `--system`.

The is a redo of [JDK-8331081](https://bugs.openjdk.org/browse/JDK-8331081), which was backed out in [JDK-8332740](https://bugs.openjdk.org/browse/JDK-8332740) due to a build failure in the microbenchmarks.

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Commit messages:
 - Merge remote-tracking branch 'origin/master' into JDK-8332744
 - Use jdk.internal.misc.Unsafe instead of sun.misc.Unsafe in microbenchmarks to avoid a sunapi warning
 - [REDO] 'internal proprietary API' diagnostics if --system is configured to an earlier JDK version

Changes: https://git.openjdk.org/jdk/pull/19397/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19397&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332744
  Stats: 159 lines in 20 files changed: 132 ins; 0 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/19397.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19397/head:pull/19397

PR: https://git.openjdk.org/jdk/pull/19397