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

[Andy Herrick]

Before signing, unsign all executables and libraries (not just those in runtime).
Also, run individual file sign and unsign commands in quiet mode.
Also, add test case to SigningAppImageTest to test that signing app that has additional launcher works, and results in validly signed launcher and additional launcher (original reported problem).

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

Changes: https://git.openjdk.java.net/jdk/pull/6636/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6636&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276837
  Stats: 64 lines in 3 files changed: 33 ins; 5 del; 26 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6636.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6636/head:pull/6636

PR: https://git.openjdk.java.net/jdk/pull/6636