gnu: Add bootstrap-binaries for 'aarch64-linux'.

gnu: Add bootstrap-binaries for 'aarch64-linux'.

These bootstrap-binaries were built against commit
8f8f250bdca917b3ce38aa0902f01b19081859a4.

* gnu/packages/bootstrap/aarch64-linux/bash,
gnu/packages/bootstrap/aarch64-linux/mkdir,
gnu/packages/bootstrap/aarch64-linux/xz,
gnu/packages/bootstrap/aarch64-linux/tar: New files.

* gnu/local.mk (bootstrap_aarch64_linuxdir)
(dist_bootstrap_aarch64_linux_DATA)
(nodist_bootstrap_aarch64_linux_DATA): New variables.
(DISTCLEANFILES): Add $(nodist_bootstrap_aarch64_linux_DATA).
(gnu/packages/bootstrap/aarch64-linux/guile-2.0.14.tar.xz): New target.
* build-aux/download.scm (filename->uri): Add aarch64-linux entry.
* gnu/packages/bootstrap.scm (raw-build): Use guile-2.0.14.tar.xz on
aarch64-linux.
(glibc-dynamic-linker, %bootstrap-coreutils&co, %boostrap-binutils)
(%bootstrap-glibc, %bootstrap-gcc): Add aarch64-linux cases.
* m4/guix.m4 (GUIX_SYSTEM_TYPE): Add aarch64 case.
(GUIX_ASSERT_SUPPORTED_SYSTEM): Add aarch64-linux to supported list.
* doc/guix.texi (GNU Distribution): Add aarch64-linux to the list of
supported systems.
* tests/packages.scm (package-search-derivation, snippet): Add aarch64
case.