package.py: fix Darwin support

package.py: fix Darwin support

- 'subprocess.Popen([d.expand("${HOST_PREFIX}otool)' requires text-mode (a more
  readable alias for the universal_newlines parameter), since otool produces
  text and the code 'out.split("\n")' expects a string, not a bytes object.
  otool is used on MacOS only, so this error isn't triggered on Linux.

- use 'startswith("darwin")' in order to support all darwin versions and not
  just specific versions (meta-darwin supports darwin21 at the moment).

Signed-off-by: Dominik Schnitzer <dominik@snap.com>
Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>