Introduce and use gnat_version_compare

Introduce and use gnat_version_compare

While testing a modified GNAT, I found that this test in
fun_renaming.exp was returning 0 for GCC 13:

    if {[test_compiler_info {gcc-6*}]}

This patch introduces a new, more robust way to check the GNAT
compiler version, and changes the gda.ada tests to use it.  A small
update to version_compare was also needed.

Note that, in its current form, this new code won't really interact
well with non-GCC compilers (specifically gnat-llvm).  This doesn't
seem like a major issue at this point, though, because gnat-llvm
doesn't properly emit debuginfo yet, and when it does, more changes
will be needed in these tests anyway.

Reviewed-by: Keith Seitz <keiths@redhat.com>