Add .hgignore file.

List all generated or temp files in .hgignore, so they
don't clutter the screen when running hg status.
(transplanted from befdb88324e18e1e80260906d0459a56ea0eb37a)

 reporter_name="Matthias KAEHLCKE"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-08/msg00055.html"

 reporter_comment=""