lang/gnat15: New GCC-15 Ada Port

GCC-15 Ada Port:

• Add GNAT-15 to the Ports Tree
• 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. The aim here is to reduce work updating the Port, or using the pkg-plist to bootstrap the next major release
• Complete ${LICENSE} block
• Expand pkg-message and make UCL compliant
• Modernise and update ${COMMENT}, ${WWW} and pkg-descr

https://gcc.gnu.org/gcc-15/changes.html#ada

PR: 292708
Co-authored-by: Alastair Hogge <agh@riseup.net>
Co-authored-by: Marcin Cieślak <saper@saper.info>