RFR: JDK-8276837: [macos]: Error when signing the additional launcher

[Andy Herrick]

- We need to unsign all executables and libraries in the app-image before signing. (not just those in the runtime).
 - Clean up excessive output by executing the individual file sign and unsigning commands in quiet mode.
 - Add conditions in SigningAppImageTest to test signing of additional launchers.

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Commit messages:
 - JDK-8276837: [macos]: Error when signing the additional launcher
 - JDK-8276837: [macos]: Errorwhen signing the additional launcher

Changes: https://git.openjdk.java.net/jdk/pull/6654/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6654&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276837
  Stats: 52 lines in 3 files changed: 21 ins; 5 del; 26 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6654.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6654/head:pull/6654

PR: https://git.openjdk.java.net/jdk/pull/6654