RFR: 8332744: [REDO] 'internal proprietary API' diagnostics if --system is configured to an earlier JDK version [v4]

[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.

Liam Miller-Cushon has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Update another reference to sun.misc.Unsafe
 - Merge remote-tracking branch 'origin/master' into JDK-8332744
 - Migrate another use of sun.misc.Unsafewq
 - Merge remote-tracking branch 'origin/master' into JDK-8332744
 - 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:
  - all: https://git.openjdk.org/jdk/pull/19397/files
  - new: https://git.openjdk.org/jdk/pull/19397/files/aa688777..a56bc3a5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19397&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19397&range=02-03

  Stats: 259097 lines in 2770 files changed: 159963 ins; 73418 del; 25716 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