hooks/prepare-32bit: new variables to improve 32bit pkg creation.

- lib32depends: if set, 32bit pkg will use this rather than "depends".
- lib32disabled: if set, no 32bit pkg will be created.
- lib32mode:
  * if unset only files for libraries will be copied.
  * if set to "full" all files will be copied

Still not perfect but should be enough to have skype-32bit.