Clang versions before v11.0 insist on having the %rX or %cX register
names instead of just a number. Since our Travis-CI is currently
still using Clang v6.0, we have to fix this to avoid failing jobs.

Message-Id: <20210512171550.476130-2-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>