RFR: 8307168: Inconsistent validation and handling of --system flag arguments [v3]

[Christian Stein]

> Please review these changes that fix a bug in `javac`'s validation and handling of `--system` flag arguments.

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Reverted change to internal `isCurrentPlatform` helper

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16264/files
  - new: https://git.openjdk.org/jdk/pull/16264/files/48d3c04e..b155a4e4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16264&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16264&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/16264.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16264/head:pull/16264

PR: https://git.openjdk.org/jdk/pull/16264