gdb.dlang/demangle.exp: update expected output for _D8demangle4testFnZv

gdb.dlang/demangle.exp: update expected output for _D8demangle4testFnZv

Since commit ce2d3708bc8b ("Synchronize binutils libiberty sources with
gcc version."), I see this failure:

    demangle _D8demangle4testFnZv^M
    demangle.test(typeof(null))^M
    (gdb) FAIL: gdb.dlang/demangle.exp: _D8demangle4testFnZv

The commit imported the commit 0e32a5aa8bc9 ("libiberty: Add support for
D `typeof(*null)' types") from the gcc repository.  That commit includes
an update to libiberty/testsuite/d-demangle-expected, which updates a
test for the exact same mangled name:

     _D8demangle4testFnZv
    -demangle.test(none)
    +demangle.test(typeof(null))

I don't know anything about D, but give that the change was made by Iain
Buclaw, the D language maintainer, I trust him on that.

Fix our test by updating the expected output in the same way.

Note: it's not really useful to have all these D demangling tests in the
GDB testsuite, since there are demangling tests in libiberty.  We should
consider removing them, but we first need to make sure that everything
that is covered in gdb/testsuite/gdb.dlang/demangle.exp is also covered
in libiberty/testsuite/d-demangle-expected.

Change-Id: If2b290ea8367b8e1e0b90b20d4a6e0bee517952d