perl-cross: bump to version 0.8.5 and move to github

The perl-cross tarball allows to patch the Perl distribution (in a
_POST_EXTRACT_HOOKS).  The github helper gives a copy of the project which
allows to build this tarball.  So, the github helper can not be used here.

[Peter: mention the change to github]
Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>