This involved:
* Installing miniperl as $dev/bin/perl
* Setting miniperl to take INC from
lib/perl5/{site_perl/,}cross_perl/${version} as well as
lib/perl5/{site_perl/,}/${version}/${runtimeArch}, in that
order. miniperl taking from runtimeArch is not really correct, but
it works in some pure-perl cases (e.g. Config.pm) and can be
overridden with the cross_perl variant.
* Installing perl-cross's stubs into
$dev/lib/perl5/cross_perl/${version}
* Patching MakeMaker.pm to gracefully degrade (very slightly) if B.pm
can't be loaded, which it can't in cross-compilation.
* Passing the right build-time and runtime perls to Makefile.PL
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