cpu: Provide a proper prototype for target_words_bigendian() in a header

cpu: Provide a proper prototype for target_words_bigendian() in a header

We've got three places already that provide a prototype for this
function in a .c file - that's ugly. Let's provide a proper prototype
in a header instead, with a proper description why this function should
not be used in most cases.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>