Packr 2.7.0 launcher executable fixes (temp transition back to libGDX)

1. Fixed a Gradle script error where it was bundling the release builds with debug info on Linux and macOS.
   • For Linux this reduces the executable size from ~722K to ~95K.
2. Compile with -no-pie on Linux to work around a Nautilus bug.
   • https://stackoverflow.com/questions/41398444/gcc-creates-mime-type-application-x-sharedlib-instead-of-application-x-applicati
   • https://stackoverflow.com/questions/34519521/why-does-gcc-create-a-shared-object-instead-of-an-executable-binary-according-to?noredirect=1&lq=1
3. Added compile flags /Os, /Gw, /Gy on Windows.
   • Combined with the new linker flags, this reduced the executable size.
4. Added /opt:icf, /opt:ref linker flags on Windows.
   • Combined with the new compiler flags, this reduced the executable size.
5. Updated Gradle wrapper to version 6.5.1.

Previous releases

As we work towards updating this repository, there are previous releases and notes available on my fork: https://github.com/karlsabo/packr/releases. This release is here for folks to try out the new changes while we get the code further updated to publish under libGDX URLs.

Maven artifacts

It's a work in progress to publish the artifacts to Maven Central under com.libgdx.packr. Version 3.0.0 should publish under those Maven Coordinates.