Make openafs-modules-source arch: all

Make openafs-modules-source arch: all

* Make openafs-modules-source architecture-specific (arch: any instead
  of arch: all).  The new method for generating the kernel source tree
  only generates correct build machinery for the architecture on which
  it was built, making the package architecture-specific.  The generated
  tree is so much smaller that this still only uses about 1MB of archive
  space more than the old method and makes each openafs-modules-source
  package much smaller.  This is a workaround until we can fix
  libafs_tree to include enough build machinery to handle all Linux
  architectures from one source tree.  (Closes: #516023)