lang/gnat12: libgomp support; sync with GNAT-15

The GNAT-15 addition to the Ports Tree included some improvements to how
the pkg-plist is maintained, updated the pkg-descr, expanded the
pkg-message, and more. Sync GNAT-12 with those modifications.

• Add xz:threads option to ${TAR} to make use of multi-threaded compression
• Add GNU OpenMP libraries. This is required for GNATColl Bindings
• Add GNAT and GNAT_SO_VERSION to ${PLIST_SUB} to remove hard-coded version dependent information.
• Complete ${LICENSE} block
• Expand pkg-message and make UCL compliant
• Modernise and update ${COMMENT}, ${WWW} and pkg-descr
• Delegate maintainership to ada@
  
  PR: 296512