RFR: 8292391: Add support for optional signing of native libraries

[Kevin Rushforth]

This PR enables an optional signing step inserted into the build right after the step to strip binaries that was added by [JDK-8278260](https://bugs.openjdk.org/browse/JDK-8278260). As is the case with the `strip` step, the optional signing is only ever enabled for production builds when running with `gradle -PCONF=Release`.

This is an optional step, meaning that `codeSignCmd` and `codeSignArgs` need to be provided by scripts external to the repo in order to enable it. By default, they are not defined, so the additional logic added by this PR will do nothing.

NOTE: as with the fix for [JDK-8278260](https://bugs.openjdk.org/browse/JDK-8278260), this PR adds three copies of the build logic. I filed [JDK-8292506](https://bugs.openjdk.org/browse/JDK-8292506) to refactor both of them (strip and sign), and to look for other duplication as well.

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Commit messages:
 - 8292391: Add support for optional signing of native libraries

Changes: https://git.openjdk.org/jfx/pull/871/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=871&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292391
  Stats: 54 lines in 1 file changed: 54 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/871.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/871/head:pull/871

PR: https://git.openjdk.org/jfx/pull/871