[gdb/ada] Fix gdb.ada/dynamic-iface.exp with gcc 7

[gdb/ada] Fix gdb.ada/dynamic-iface.exp with gcc 7

This test in test-case gdb.ada/dynamic-iface.exp passes with gcc 8:
...
(gdb) print obj^M
$1 = (n => 3, a => "ABC", value => 93)^M
(gdb) PASS: gdb.ada/dynamic-iface.exp: print local as interface
...
but fails with gcc 7:
...
(gdb) print obj^M
$1 = ()^M
(gdb) FAIL: gdb.ada/dynamic-iface.exp: print local as interface
...

More concretely, we have trouble finding the type of obj.  With gcc 8:
...
$ gdb -q -batch main -ex "b concrete.adb:20" -ex run -ex "ptype obj"
  ...
type = <ref> new concrete.intermediate with record
    value: integer;
end record
...
and with gcc 7:
...
type = <ref> tagged record null; end record
...

The translation from tagged type to "full view" type happens in
ada_tag_value_at_base_address, where we hit this code:
...
  /* Storage_Offset'Last is used to indicate that a dynamic offset to
     top is used.  In this situation the offset is stored just after
     the tag, in the object itself.  */
  if (offset_to_top == last)
    {
      struct value *tem = value_addr (tag);
      tem = value_ptradd (tem, 1);
      tem = value_cast (ptr_type, tem);
      offset_to_top = value_as_long (value_ind (tem));
    }
...
resulting in an offset_to_top for gcc 8:
...
(gdb) p offset_to_top
$1 = -16
...
and for gcc 7:
...
(gdb) p offset_to_top
$1 = 16
...

The difference is expected, it bisects to gcc commit d0567dc0dbf ("[multiple
changes]") which mentions this change.

There's some code right after the code quoted above that deals with this
change:
...
  else if (offset_to_top > 0)
    {
      /* OFFSET_TO_TOP used to be a positive value to be subtracted
	 from the base address.  This was however incompatible with
	 C++ dispatch table: C++ uses a *negative* value to *add*
	 to the base address.  Ada's convention has therefore been
	 changed in GNAT 19.0w 20171023: since then, C++ and Ada
	 use the same convention.  Here, we support both cases by
	 checking the sign of OFFSET_TO_TOP.  */
      offset_to_top = -offset_to_top;
    }
...
but it's not activated because of the 'else'.

Fix this by removing the 'else'.

Tested on x86_64-linux, with gcc 7.5.0.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29057