gar.conf.mk: add GCC12 compiler block, switch GARCOMPILER default path
- Add GCC12 to GARCOMPILERS; add GCC12_{CC,CXX,F77,FC,*_FLAGS,CC_VERSION} vars
- PATH and BUILD_PATH now use $(GARCOMPILER)_CC_HOME instead of hardcoded GCC9
- garrc sets GARCOMPILER=GCC12, GCC12_CC_HOME=/opt/ozsw
Initial import of OZSW mgar build tree