[gdb/testsuite] Add KFAIL for PR ada/30908

[gdb/testsuite] Add KFAIL for PR ada/30908

With gcc 13.2.1, I run into a cluster of fails:
...
FAIL: gdb.ada/str_binop_equal.exp: print my_str = "ABCD"
FAIL: gdb.ada/widewide.exp: print my_wws = " helo"
FAIL: gdb.ada/widewide.exp: print my_ws = "wide"
...

The problem is that the debug info contains information about function
ada.strings.maps."=", and gdb uses it to implement the comparison.
The function is supposed to compare two char sets, not strings, so gdb
shouldn't use it.  This is PR ada/30908.

I don't see the same problem with gcc 7.5.0, because the exec doesn't contain
the debug info for the function, because the corresponding object is not
linked in.  Adter adding "with Ada.Strings.Maps; use Ada.Strings.Maps;" to
gdb.ada/widewide/foo.adb I run into the same problem with gcc 7.5.0.

Add KFAILs for the PR.

Tested on x86_64-linux:
- openSUSE Leap 15.4 (using gcc 7.5.0), and
- openSUSE Tumbleweed (using gcc 13.2.1).

Approved-By: Tom Tromey <tom@tromey.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30908