Fix gdb.ada/arrayptr.exp results

PR ada/28115 points out that gdb.ada/arrayptr.exp works with GNAT 12,
but fails with minimal encodings in earlier versions.

This patch updates the test to try to report the results correctly.  I
tried this with the Fedora 34 system gcc (GCC 11) and with a GCC 12
built from git trunk sometime relatively recently.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28115