build: Split Makefile.am into two parts.

* Makefile.am (MODULES): Move gnu/* to gnu-system.am.
  (patchdir, dist_patch_DATA, bootstrapdir,
  bootstrap_x86_64_linuxdir, bootstrap_i686_linuxdir,
  dist_bootstrap_x86_64_linux_DATA, dist_bootstrap_i686_linux_DATA,
  nodist_bootstrap_x86_64_linux_DATA, nodist_bootstrap_i686_linux_DATA,
  install-data-hook, DISTCLEANFILES, DOWNLOAD_FILE,
  gnu/packages/bootstrap/x86_64-linux/guile-2.0.7.tar.xz,
  gnu/packages/bootstrap/i686-linux/guile-2.0.7.tar.xz): Move to...
* gnu-system.am: ... here.  New file.