Fix GNAT version check in gdb.ada

Fix GNAT version check in gdb.ada

Commit 1411185a ("Introduce and use gnat_version_compare") changed the
Ada tests to use a new proc for version checking.  Unfortunately this
patch inadvertently reversed the sense of the test in
packed_array_assign.exp.

After fixing this, I went through that patch again and looked for
other problems.  I found one spot where the wrong syntax was used, and
some others where I believe the sense of the test was inverted.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32444
Approved-By: Andrew Burgess <aburgess@redhat.com>