Perl cross support

This branch is an initial implementation of debhelper dh_auto_* support for cross building Perl XS modules.

As discussed in the thread around https://lists.debian.org/debian-perl/2019/10/msg00015.html perl/5.30.0-7 added the (currently virtual) package perl-xs-dev providing the host architecture Config.pm in /usr/lib//perl/cross-config- for cross builds.

The proposed changes should be no-ops for native builds, and no-ops with a warning for cross builds when the host architecture Config.pm is not available (even though those are currently broken anyway, so they fail later.)

I'm able to cross build a number of both ExtUtils::MakeMaker and Module::Build based simple packages with this and trivial build dependency changes (mainly 'perl' -> 'perl:native, perl-xs-dev' and some <!nocheck> annotations), but it's of course possible that the implementation will need tuning in the future if we discover common requirements in more complex packages.

Any comments are welcome of course.