[Linux] Build and deliver the libav media stubs for openjfx build

The open build of JavaFX for Linux compiles and links against the currently installed version of libabformat, and as such, will only run against that version.

The closed build will create stub .so files for several different versions. We do this by compiling and linking against pre-built binaries that we host on an internal server.

In order to enable building a version of FX for Linux that will run on multiple versions of Linux we need to either make the prebuilt binaries available via maven central (which is what we used to do for three of the WebKit libraries), or we need some other solution.