Assume an unrecognized gnatmake is very new

I don't know why -- though I suspect there may have been a change to
the output of "gnatmake --version" -- but recently some gdb.ada tests
have stopped running when llvm-gnatmake is used.

The llvm-gnatmake I am testing against prints a version string that
isn't recognized by gnat_version_compare.  However, it seems to me
that AdaCore is probably the main place where this can even occur; and
furthermore that without some extra work, it seems reasonable for
gnat_version_compare to assume that an unrecognized gnatmake is "very
new".