[lldb] Fix memory tag unsupported test

This corrects the test added in
31f9960c
and temporarily patched in
3b4aad11.

This test checks that the memory tag read
command errors when you use it on a platform
without memory tagging.
(which is why we skip the test if you actually
have MTE)

The problem with this test is that there's
two levels of unsupported each with it's own
specific error.

On anything that isn't AArch64, there's no
tagging extension we support. So you're told
that that is the case. As in "this won't ever work".

When you're on AArch64 we know that MTE could
be present on the remote and when we find that it
isn't, we tell you that instead.

Expect a different error message on AArch64 to fix
the test.