gnu: Add bootstrap binaries for 'armhf-linux'.

* gnu/packages/bootstrap/armhf-linux/bash,
  gnu/packages/bootstrap/armhf-linux/mkdir,
  gnu/packages/bootstrap/armhf-linux/tar,
  gnu/packages/bootstrap/armhf-linux/xz: New files.

* gnu-system.am (bootstrap_armhf_linuxdir, dist_bootstrap_armhf_linux_DATA)
  (nodist_bootstrap_armhf_linux_DATA): New variables.
  (DISTCLEANFILES): Add $(nodist_bootstrap_armhf_linux_DATA).
  (gnu/packages/bootstrap/armhf-linux/guile-2.0.11.tar.xz): New target.
* build-aux/download.scm (file-name->uri): Use newer date in URI for
  armhf-linux.
* gnu/packages/bootstrap.scm (raw-build): Use "guile-2.0.11.tar.xz" on
  armhf-linux.
  (glibc-dynamic-linker, %bootstrap-coreutils&co, %bootstrap-binutils)
  (%bootstrap-glibc, %bootstrap-gcc): Add armhf-linux cases.

* m4/guix.m4 (GUIX_SYSTEM_TYPE): Add armhf case.
  (GUIX_ASSERT_SUPPORTED_SYSTEM): Add armhf-linux to list of
  supported systems.
* doc/guix.texi (GNU Distribution): Add armhf-linux to the list of
  supported systems.