perlPackages: Add cross-compilation support.

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