[gdb/ada] Fix gdb.ada/overloads.exp on s390x

[gdb/ada] Fix gdb.ada/overloads.exp on s390x

On s390x-linux, with test-case gdb.ada/overloads.exp and gcc 7.5.0 I run into:
...
(gdb) print Oload(CA)^M
Could not find a match for oload^M
(gdb) FAIL: $exp: print Oload(CA)
...

The mismatch happens here in ada_type_match:
...
      return ftype->code () == atype->code ();
...
with:
...
(gdb) p ftype->code ()
$3 = TYPE_CODE_TYPEDEF
(gdb) p atype->code ()
$4 = TYPE_CODE_ARRAY
...

At the start of ada_type_match, typedefs are skipped:
...
  ftype = ada_check_typedef (ftype);
  atype = ada_check_typedef (atype);
...
but immediately after this, refs are skipped:
...
  if (ftype->code () == TYPE_CODE_REF)
    ftype = ftype->target_type ();
  if (atype->code () == TYPE_CODE_REF)
    atype = atype->target_type ();
...
which in this case makes ftype a typedef.

Fix this by using ada_check_typedef after skipping the refs as well.

Tested on x86_64-linux and s390x-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR ada/32409
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32409