gnu: coreutils: Fix cross-compilation issue with 'dummy-man'.

This should work around the bug described at
<http://git.yoctoproject.org/cgit.cgi/poky/plain/meta/recipes-core/coreutils/coreutils-8.22/fix-for-dummy-man-usage.patch>.

* gnu/packages/base.scm (coreutils)[patches]: Add
  'coreutils-dummy-man.patch'.
  [native-inputs]: Don't pass PERL when (%current-target-system) is
  true.
* gnu/packages/patches/coreutils-dummy-man.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.