gnu: add bootstrap support for mips64el-linux using the MIPS N32 ABI.

gnu: add bootstrap support for mips64el-linux using the MIPS N32 ABI.

* .gitignore: Add entry for bootstrap/mips64el-linux/guile-2.0.9.tar.xz.

* gnu-system.am (bootstrap_mips64el_linuxdir,
  dist_bootstrap_mips64el_linux_DATA, nodist_bootstrap_mips64el_linux_DATA):
  New variables.
  (DISTCLEANFILES): Add $(nodist_bootstrap_mips64el_linux_DATA).
  (gnu/packages/bootstrap/mips64el-linux/guile-2.0.9.tar.xz): New target.

* build-aux/download.scm (file-name->uri): Adapt to the newer mips64el-linux
  bootstrap tarballs.

* gnu/packages/bootstrap.scm (%bootstrap-guile, %bootstrap-coreutils&co,
  %bootstrap-binutils, %bootstrap-glibc, %bootstrap-gcc): Adapt to the newer
  mips64el-linux bootstrap tarballs.

* gnu/packages/bootstrap/mips64el-linux/bash:
* gnu/packages/bootstrap/mips64el-linux/mkdir:
* gnu/packages/bootstrap/mips64el-linux/tar:
* gnu/packages/bootstrap/mips64el-linux/xz: New files.