gnu: coreutils: Move Perl to 'native-inputs'.

* gnu/packages/base.scm (coreutils): Add 'native-inputs' field.
  This should allow cross builds to have man pages, and work around this
  'dummy-man' bug:
  <http://git.yoctoproject.org/cgit.cgi/poky/plain/meta/recipes-core/coreutils/coreutils-8.22/fix-for-dummy-man-usage.patch>.